If there was an aftermarket radio in place, then a voltage divider circuit was wired to send approximately 5 volts through the factory amp turn on wire in the smaller harness behind the radio. Check to see this was done correctly or if the wiring is faulty. The amps for for front woofers in the door panels are located under the driver side dash on the left. You can disconnect the amps to further diagnose the problem. Here are wiring diagrams that might help.