There's a chance that this could work for other USB devices (maybe an iPhone too, but I don't have one), but there's also a chance that this could fry your device, car, or perhaps eat all of the cheese in the house. If you're not confident, or not competent, you probably don't want to try this. I accept no responsibility for any bad things happening.
If you're vaguely interested in stuff like this, you should probably check out ladyada's MintyBoost! kit, which helped prod me towards actually doing this.
Step 1: Bits Needed
If you want the very simple steps without any instructions, all I did was connect a 27k Ohm resistor from V+ (Pin 1) to D- (Pin 2), then another 12k Ohm between D- and D+ (Pin 3). Dead easy.
Step 2: Open the Charger
Take the fuse and cap out. Then you need to split the charger open. Mine was welded together so I had to saw through both sides of the case and then lever it open. Don't cut too deep with the saw or you'll end up going through the components of the charging circuit.
Step 3: Add the Resistors
According to the USB charging and power spec I should be able to simply short the two central pins (the data lines) and the device will detect that it's connected to a charging device and start charging.
I tried this first of all, but it didn't work (the iPod detected it had been plugged in to something, but refused to charge). I knew the iPod charged from my MintyBoost!, and after a quick look at the schematic and some playing with the multimeter, I decided it would be worthwhile adding a couple of pull-up resistors.
I don't think the values are that vital (the USB spec is quite forgiving), but I decided to try matching roughly what the MintyBoost! was giving, apart from the V+ to D- resistor, which was guesswork.
Don't forget to check that you've got the correct resistance between the pins, and that they're not connected to anything they shouldn't be!
Step 4: Test (and Rejoice)!
Next I tried all three iPods I could get my hands on. Success!
I wrapped the charger with duct tape to complete that "finished" look.