the first thing you need to do is buy a multimeter. they are around 8-10 bucks..INVEST IN ONE!
my best guess as to where the problem lies is the ground. its very easy to ground to a metal surface which is somehow insulated from the cars chassis. so...heres my advice:
1) unplug the RCAs from the amp. if the amp doesnt recieve a good ground reference from where it ought to, it may try grounding through the shield on the RCAs...which endagers the headunit.
2) hook up the ground and power wires. use a multimeter to probe the connections (put the red probe to the positive termanal and the black one to the negative terminal) to ensure that there is indeed a voltage reference between the two (use the DC voltage setting to probe this BTW) if not, find another ground.
3) once you read a nice 12+v reading, hook up the remote turn-on lead. now turn on the headunit (leave the RCAs unattached). it should light up. if not, probe the remote wire the same way you did the power wire. if the remote wire is dead the amp wont turn on.
4) if it still wont turn on, run a jumper wire from the positive terminal to the remote turn-on terminal (you can use a small guage wire). it should light up immediatly. if not, the amp is broken. do NOT leave the amp hooked up in this fashion, as it will drain the battery. its only for testing purposes.
as soon as the amp lights up (no matter what the step), reconnect the RCA input cables. everything should be fine.
