Well obviously the problem is in the Head unit. You probably will have blown some protection diodes of sorts, but with surface mount technology, who knows. 1 thing for sure, it'll have to be opened up for a lookee and unless you have some nice tech gear you prolly won't want to attempt it yourself. The bits won't cost anything it's the labour which will be the killer, but it shoudn't take a quality tech more n a hour.
but like toyoda said, if it's a newbie, take it back.