Aslong as the car is fairly new, and both car and stereo use ISO connectors, and you aren't doing anything "non standard" (i.e. amps, extra speakers etc etc), you'd have to be a bit of a gorilla to short anything out.
My missus recently brought one of these (not sure its the EXACT model) it was £89 when she got it, and its pretty decent, plays DVD's full of mp3's, and has a USB slot so you can plug in a USB storage device, and also an SD slot and mp3 jack.
If, by any slim chance, I am on at the same time as anyone else, hit me up and I'll race.
As for public racing, I've been pretty lucky, and have mainly had very good races, sure you get a couple of tools here and there, but they quickly get bored.