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This will be the repair info, the idea being to cut down on general repair questions by providing an idea of what to look for. Modify as needed, if someone has a good system for fault finding, please put it up. Feel free to scrap my entire part, i'm not that happy with it myself :)
First up, repairing a car amp, in my opinion, falls into 2 catagories: Dead simple, and throw it out.
Dead simple is when you have shorted it, and usually end up blowing the output transistors. You may also have done the power supply mosfets. This is basically go through the amp with a multimeter, see if the transistors or mosfets are shorted (some are absolute zero, i've had transistors say 100 ohms as well). These are the large packaged devices bolted to the heatsinks. Usually you can replace them and the amp is ok, as long as when they died, nothing else got damaged.
A good troubleshooter is a variable bipolar supply. A transformer, rectifier bridge, a pair of caps, and bleeder resistors with a metered variac. Attach this to the rails for the amp section and bring it up SLOWLY! Watch for any excessive current draw as you bring the voltage up. Do not exceed the voltage of the power supply caps on the amp board! With the voltage applied, check for a d.c. offset on the output stages. Doing this allows you to verify the amplifier section is working before you fry the power supply section a second time because the amp section was shorted.
Some amps do not turn on the driver section unless the turn on voltage is applied to the trigger lead. Most will come up and will play through speakers on this setup.