WEll you just need the propper DC-DC adapter.
The wall plug most likely is a transformer which takes the AC and turns it into (since its a USB unit) most likely 5v
You could take the 5v from the computers psu or get another DC-DC adapter for that purpose alone.
Note, if the original adapter outputs 12v, DO NOT connect it straight to the cars power line. It is UNREGULATED and can vary from 10 to 14 volts potentially frying the unit.
Either get the power from the PSU (which is regulated) or other regulated DC-DC adapter.
As for getting it to the unit. simply splice the wire from the original adapter, take the output from the DC-DC adapter and send it to the spliced wire.
If you dont want to cut up the original piece, head over to radioshack and see if they have the same type of plug in stock and use that.