Introduction: USB-ports in Your Car

I was tired of mucking about with cigarette lighter-adaptors, extension cords and power splitters, just to power my navigation system, dashcam and charging our mobile phones, which all run on USB power. Neither me nor my girlfriend smoke, so the ashtray in my car has had nothing to do since the car was bought years ago (the cigarette lighter itself is brand-spanking new as well), so I decided to give it purpose.

The end result is an ashtray with four USB charging ports and a simple car battery monitor, which are invisible when not in use.

If you want to make this yourself: you'll be looking at about two hours of work on making it pretty and €40-something in parts: two 12V-to-USB adaptors, a battery monitor, some wiring, connections and hardware. The green LED in the centre of the tray is the battery monitor which keeps track of my car battery and charging circuits: green when OK, blinky green when charging and OK, red when not OK, and blinky red when seriously not OK.
This monitor is a kit by Velleman and sets you back €6 if you build it yourself, which I did, or €9 for the soldered PCB. I also used 2 cigarette plug adaptors with SMPS-circuitry and high-powered ports (€12 each, ymmv), some bits of wire, a set of connectors with wires attached so I can detach the tray when in need of expansion or modification, some shrink tubing, some nuts and bolts and a pair of branch connectors to hook the whole contraption up to the existing wire loom leading to the cigarette lighter.

First, test everything before you take it apart! I will not be responsible for you voiding your warranty on a DOA bit of kit.
Liberate the adaptor's PCB's from their enclosures, note the polarity and desolder any excess hardware: mine came with the lighter contacts directly soldered to the PCB. I didn't need those, so I got rid of them.
You might also want to take a look at the USB socket  and make sure they are securely soldered to the PCB. Mine weren't, which may lead to bad contacts later on.
Solder a couple of wires to the PCB of at least 0.75qmm. This is more a warning to not use kynar wire than a requirement for thick wire, but currents of up to 3 amps in total will flow and if you're a little tech-savvy you get the point. Your connection will be more rugged, too.
Assemble the battery monitor if you want one and test it as well.

How to build it.
Well, that's entirely up to you and largely depends on where you want to put it. Like I said, I put it all in the ashtray. The PCB's of the adapters I used fit perfectly on the sides of the tray and were just small enough to rest on the bottom of it.
I drilled a few holes for nuts and bolts and the wires coming out of the tray. Make sure to arrange the wires in a fashion that they are able to move and avoid them resting against sharp edges.
Before you hook everything up, make sure you find a wire that's without power when you switch your car's engine off and keep an eye on polarity.

Additions and trim.
While everything works as expected, there is always room for improvement. The modular setup of this build allows you to add more things if you want to, but I'm quite happy with how it turned out. I'm going to dim the blue LED's a bit; not that the entire car is fllooded in a blue glow at night, but they do emit a large amount of light. Some work with an Edding or Sharpie should take care of that.
A cover is also on my list, but first to see whether I want to add another set of ports or not.