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.