First thing to do is make SURE it is not your fault. Have somebody bench test it if possible, or test it in another vehicle that has amps working properly. If it STILL doesn't work, talk to him about having it warrantied. Make sure he is NOT going out of town for any reason, if he says "I am leaving or I will be unavailable" at all either keep him around or get some other kind of involvement very soon. The first thing to do would be to call the police about fraud if he is totally unhelpful, after the police do some level of report you can probably contact a lawyer. Take him to court (this is all assuming that you have talked to the guy and he won't help you out at all).
Chances are he will at least be able ot do something, and don't take any "It will take X weeks" for an answer, he is liable for his faulty product...
-hoka