When I was about 75% done with my system, I stumbled upon this instructable. It's pretty similar to my setup, and I ended up using his source for some of my tubing connectors, www.usplastic.com, and his idea for the quick release connectors.
Step 1: Parts and Tools Needed
Some abbreviations used (ID = inner dimension, OD = outer dimension, VDC = volts direct current, VAC = volts alternating current)
• Plastic hard-sided cooler - In my case I went with a smallish, 12-can cooler made by Igloo, ($9.88 at Walmart.)
• 12VDC submersible bilge pump - A bilge pump is going to be the 12VDC that you'll need to run it in your car. Otherwise, you'll need to find a 12VDC fountain pump (usually sold as a "solar" pump.) Alternately, you could get a power inverter to run a standard 120VAC fountain pump from your cigarette lighter socket, which can be noisy and produce a lot of heat - unless you're willing to spend a lot of money on a really good one.
• Cigarette lighter plug power adaptor
• ~5 feet of 12VDC wire
• 4 or 5 refreezable freezer packs
• 3" of 3/4" ID vinyl tubing
• 20-24" of 3/8" ID vinyl tubing
• 14" of 1/4" ID vinyl tubing
• 30-35' of 1/4" OD tubing (use drip irrigation tubing)
• (1) 3/4" X 3/8" ID reducer coupling
• (2) Y adapters for 1/4" ID tubing
• (4) 1/4" X 5/32" reducer couplings
• (2) small screw-type hose clamps - or use 3/8" X 1/4" reducer couplings
• (2) 3/8" quick release couplings
• (1) 12" X 18" sheet "Darice Mesh" or "plastic canvas" - get the extra stiff kind
• spool of cut-to-length, plastic-coated twist ties
• 1/2 yard of woven cotton fabric (optional)
• felt fabric to cover bottom of cooler (optional)
• distilled water
• utility knife
• duct tape
• electricians tape, or wire-splicing connectors
• drill with 1/2" bit and 3/16" bit
• screwdriver (common head)
• hot glue gun
• sewing machine (optional)
Step 2: Creating the Cooling Coils
Take a Sharpie or other felt-tip pen and sketch out how you want the tubing to coil onto the sheet of plastic canvas. Keep in mind that the tightest diameter you can bend this tubing into is about 2" - and you don't want to kink it. At this point, you might want to go into your car and measure out how far from the cooler your car seat pads are going to be, allowing for enough slack to tuck your tubing away. I put the cooler in the trunk and ran the tubing out the pass-through in the backseat and to each car seat, which required about 15' of tubing per seat (3' to 5' in the seat pad, 5' for the supply tube and 5' for the return tube).
Measure out this much tubing and mark it with a piece of tape or something, then at this mark, start attaching it to the plastic canvas. Get your plastic coated twisties and, starting at one end of your inked path, attach the tubing to your plastic canvas, tucking in the twist tie ends as you go. Once you've completed attaching the cooling coil to the pad, add the same amount of extra tubing for the return section back to the cooler. I'm using all plastic parts here because if there's any condensation, we don't want there to be any rust or other damage.
Creating cloth covers: This step is optional. I did it because I wanted extra protection against condensation by providing an absorptive layer, and also to protect the inside of the car seat from damage by the twisties or plastic canvas. I bought some woven fabric from the muslin section that kind of looks like waffles. I cut it so it would be an inch larger on all sides, then had my wife sew it together into a sleeve to put the cooling pads into. She also put in three snaps. My wife is very crafty! If you can't sew, you might try the peel and stick Velcro strips.
Step 3: Modifying the Cooler and Hooking Up the Pump
My bilge pump has an outlet for 3/4" ID tubing. I need to reduce this size, in stages, down to .17" ID tubing (1/4" OD). Since there isn't a reduction coupler that will do this all at once, we have to do it with a few parts. The first one reduces it from 3/4" ID to 3/8" ID (US Plastic item #064383). Attach your short piece of 3/4" ID vinyl tubing to your pump and then insert the reducer coupler to the other end. Then to the other end of the coupler, attach enough 3/8" ID vinyl tubing to go up and out the new supply hole you drilled. Make sure that about an inch of tubing sticks out of the cooler. Insert the male end of the quick release connectors (US Plastic item #060467) to the short sections of tubing emerging from the cooler.
As a side note about my pump; I bought it from some guy on Craigslist and when I fired it up, the flow was pathetically low and slow. As far as a bilge pump is concerned, he sold me a hunk of garbage. But for this purpose, the slow flow is actually a benefit. I've heard of other projects like this and how their back really freezes and they have to cycle the pump off and on to keep from going numb. I had been thinking that I would have to rig up some sort of thermostatic control or something, but as it worked out, this pump is perfect. I just have to keep the pump pretty much on the same level as the cooling pads as it will only pump about 12" of head.
Next, cut about 6 inches of 3/8" ID tubing and stick that through the other large hole, leaving about an inch sticking out of the cooler on the outside. This will be the end piece of your return tube. Next, pass the power wires for the pump out through the small hole you drilled and splice them into your 12VDC wire, which then is spliced into the cigarette lighter plug. Note: Be sure to locate your splicing outside of the cooler. You do not want to get water on bare wires!
I applied some hot glue around both tubes and the wires where they penetrate the inside surface of the cooler. This is to prevent leaks from water sloshing around as you drive. You could use silicone caulking instead of hot glue. Lastly, I cut a piece of felt and hot glued it to the bottom of the cooler to prevent it from sliding around on the carpeted floor in my car's trunk.
Step 4: The Supply and Return Tubing Assemblies
Each cooling pad has two tubes coming out of it, one for supply and one for return. It doesn't really matter which is which. Connect the supply and return tubes appropriately to the assemblies you just created.
Wrap each supply line with some fabric and then with some duct tape to act as insulation. I'm not sure how well this insulation method is working, because when I touch the supply line, it's cool to the touch. I'm open for suggestions for more effective ways of insulating 1/4" OD tubing that are also easy and cheap!
Step 5: Installation and Suggestions for Use
Cooling Pad Installation: Most, if not all, baby car seats have removable covers that are held on with an elastic edge that wraps around the whole seat. The supply lines go into the car seats under the upholstery in the front of the seat, near where the babies' feet are, and run along the fold of the seat and the side bolster. The cooling pads are right behind the baby's back.
Power: I ran the power wire up the middle of the car, along the floor and up to the cigarette lighter plug. I keep it plugged in, so that when my car is on, the pump is running.
Keeping it Cold: My method with rotating the freezer packs goes like this: I'll put 4 freezer packs into the cooler just before I go out on a drive. When I'm done for the day, I take the freezer packs out and put them back in the deep freeze. Then I take out a 5th packet from the freezer and put that one in the cooler to stay there the rest of the evening and overnight until I go driving again. Then I take out that one packet and replace it with 4 freshly frozen ones. This keeps the water in the cooler cool and doesn't let it get hot like the rest of the car's interior does. This way when I put in new packets, they don't partially melt to cool down the hot water.
The result: they work great! They keep the babies' backs cool without ever getting cold, or letting them get too warm. I think that if the pads were in direct contact with them, it would be too cold. Placing the pads behind the seat cover (which also has a thin layer of foam pad) works to remove heat from the seat, which was the problem to begin with.
Problems: One problem I’ve started seeing is that after a while, the water starts getting slightly murky and starts getting stuff in it. I’m not sure where this is coming from, but I’m wondering if I should hook up a filter to it somehow. I’ve heard someone else suggest using a little bit of bleach in the water – this may help if it’s due to something growing in there.