check the power output of the radio (hook up a known good speaker to the speaker outputs). this will let you know if the Alpine's speaker outputs are working or not. if this is good, the stock amp may be bad. im not sure if a quick and easy way to check it. My advice would be to just bypass it if the speaker outputs still work. Depending what year your car is it should perform a little better than the stock amp. You will need some sort of crossover to remove the bass frequencies from the factory speakers, or they wont last long.
hope that helps...
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