just read your eject button post, ive never seen anything like that, doesnt sound too good though.
few initial troubleshooting ideas, put the unit back in your GFs car, see if it works then. then you will know for sure if its your car or the stereo.
after that, if you can get a long length of decent quality wire ( even speaker wire would work as long as you dont turn the HU up or have it on too long) first connect the wire to both the red and yellow wires going into the stereo ( use the bullet clips i said about earlier, doesnt have to be a particularly fantastic connection or anything, just quiet secure) then run the wire round into the engine bay and straight to the battery + terminal, just be careful not to shut the doors or bonnet and make sure the wire has no splits in ( dont want any short circuits!). if it now works this will tell you it would be a good idea to wire the HU direct to the battery because it would mean the standard wiring had issues. at least thats what i would do, but i dont want you setting yourself on fire or killing your car so its up to you if you reckon you can do it safely!
the black (earth ) HU wire has caused me problems in the past but thats not too easy to check without a multimeter or cutting wires.
also i would leave it disconnected until you get it fixed, it may flatten your battery.