This was the problem that faced me: my car is old enough that it doesn't have an AUX jack to plug an iPod into. However, it's not old enough to have a cassette tape player, so I can't use one of those nifty adapters. Retrofitting it so that it does have a jack would be too expensive. The result is that I can't play music from my iPod in my car. I decided that the best solution was to build a sort of iPod dock designed to fit in the car, one that would run off of the cigarette lighter outlet and be cheaper than a regular iPod dock. I made it out of a set of old computer speakers and designed it to fit my car. Here is the result. Feel free to rate and comment. Hope you like it!

P.S. I understand that an FM transmitter is a viable solution. It has been pointed out several times in the comments. However, this is what I made, and it is merely one way of doing things. Thanks.

Step 1: Materials

The reason that this project is so easy is that it uses old computer speakers as its base. Computer speakers already have the circuitry built in, so no extra effort or amplifiers are necessary. I got a very nice pair for $5 off of Craigslist (they're normally $15).

Materials:
-Computer speakers - $5 (on Craigslist)
-Sheet plastic - $5
-Car power adapter* - $9.99 (at radioshack )
-Stereo plug - $3.99 (for a pack of two at radioshack )
-Window screen - $0 (I got mine from an old window)
-Small nuts and bolts (I had plenty of these already)

Total: $24

Tools:
-Jigsaw
-Power drill
-Hole-cutting drill bit
-Soldering materials
-Screwdriver
*A word about adapters: my speakers ran on an AC adapter that produced 12V DC at about 1.5 amps, so I got a 12V 2A adapter. There are other adapters with different outputs, so be sure to get one that fits your speakers.

Step 2: Dismantle the Speakers

First, remove any covers or screens. Then unscrew everything. If it still doesn't come apart, pry it open with a screwdriver or something. You won't be needing that plastic covering anyway. Also remove the circuit board. It's probably inside the right speaker. Cut the wire going from the audio jack to the circuit board if you can't remove it. Be sure to save the volume knob.

Step 3: Choose and Trace Your Compartment

Find a good place in the car for the two speakers. I used the compartment in the middle of the front seats. Then you have to trace the outline of that compartment. To do this I took a piece of paper, placed it over the compartment, and just sort of traced over the edges. Then, I transferred that outline to a piece of cardboard and cut it out so I could use it as a template. If anyone has a more accurate way of doing it, let me know.

Step 4: Cut the Plastic

Trace the outline of the compartment onto the plastic, which should be covered in some sort of adhesive paper, and cut it out using a jigsaw or something similar. Try to be very exact; it's important for the plastic to fit tightly into the compartment.

Step 5: Drill the Holes

You need three drill bits for this part: a large hole saw bit (mine was 2 1/2 inches), a drill bit the size of your bolts, and a smaller one for drilling pilot holes. While still keeping the paper on the plastic, place your speakers face-down on top in the position you want them to be. My compartment was too small to fit both speakers side by side, so I placed them diagonally and overlapped one corner (it looks cool anyway). Once you have them positioned, trace their outlines and mark the position of the bracket holes (the speakers should look like the one in the picture below, with a bracket attached). Find the center of your circular speaker hole by drawing lines from the corners of the rectangular-ish bracket shaper and marking their intersection. Once you have all of the holes marked, drill the large holes with the hole saw bit (be sure to keep it centered) and the small holes, starting with pilot holes and moving in to the larger bit. Plastic is a little slippery, so pay attention to keeping the drill on track.

Step 6: Attach the Speakers and the Screen

Now that you have the holes cut, the speakers can be attached. Here's where that screen comes in. Cut a section out about the size you need. This is easier if it's the kind that isn't actually metal. Trim it down so that it fits nicely and leaves room for where you're going to put the volume knob. Then poke holes for the bolts and put on the screen, the speakers, and the nuts. It should look like the last picture.
P.S. This later turned out to be an important window. I got in trouble. Oops... So make sure your window really is old.

Step 7: Two More Holes and a Circuit Board

Now you have to attach the circuit board. Mine had four important things attached to it: a volume knob, a little green LED, a plug for power, and a plug for the left speaker. The latter two will come later and face away from the plastic. The knob and the LED will stick out the front of the plastic, so they need holes. Mark where you want them and drill. Once that's done, put the circuit board in between the speakers so that everything is in the right place. I found that the easiest way to attach it is with a piece of duct tape (no one's going to see it).

Step 8: Shorten Wires and Solder

You probably had to cut a wire or two to remove the speakers from their original casing. If you didn't, you may still want to. This step is not entirely necessary, but I recommend shortening the wires, just to keep things neat. First, cut the wire that goes to the left speaker (the right speaker should already be attached to the circuit board), shorten it, and solder in the middle (this involves soldering two wires, the positive and negative inputs, which are both contained in the cable). You can insulate it and cover it with heat-shrink wrap or electrical tape.
There are two ways to deal with the audio plug: you can treat it like the other wire and shorten it by cutting some out in the middle and soldering the three wires inside, or you can attach a new plug at the end and eliminate the break. I chose the second method. For that method you need a new stereo plug. It should have a plastic cover on the bottom that unscrews. Remove it and slip it down over the wire. The plug will have three terminals: one big one at the base and two smaller ones. The cable that was connected to the original plug has three wires: one unshielded and two wrapped in regular rubber coverings. Notice a pattern? The uninsulated wire gets soldered to the large terminal on the plug—this is ground. The other two are left and right. You can figure out which is which with a little experimentation, although it doesn't really matter.
Finally, plug in the power adapter to the power jack on the circuit board.

Step 9: Install It!

Well, almost. One more important thing: since all of the contacts underneath the circuit board are exposed, it is likely that they will be touching something metal. This is bad. More than one contact is touching the same piece of metal could cause a short in the circuitry. If this is the case, insulate any suspicious exposed electronic connections with electrical tape or something.

Anyway, after testing it with the original power adapter (recommended), I simply had to install it in my car. My original stencil had been accurate and it fit nicely into the compartment. It should be set up so that the power cord comes out and is plugged into the nearest cigarette outlet (in my car there was one behind the compartment) and the audio cable comes out in front so that it can be plugged into your music player. The neat thing about this design is that you can tuck the cable into the compartment, close it, and the whole thing is hidden. To listen, open the compartment, plug in your music player, turn on the speakers by clicking on the volume knob, and rock out!