how the hell did u/he fuck up the taillight wires installing a CD player? thats prob # 1.

best way to fix wiring probs is to find where you need voltage, how much, and trace it back as far as you need to. so, if you need 12v at the taillight plugs, then check the wiring at the taillights. if you dont have 12v, either follow the wires back to the dash, or go to the next place u know the wires are good. eventually u will find a spot where u dont have the voltage you need.