Also, does the vehicle have a booster? It would be under the passenger seat.
If so, then that could be the problem. As I understand it (correct me if I’m wrong jes2) the factory amp is basically what we used to call a “booster”, in that it takes high level signals and amplifies them a bit. This is in contrast to the “amp” which takes low level inputs (preamp) and amplifies them a lot, or takes high level signals, turns them down low and then amplifies them (ala Kicker 200.4). They are both amplifiers, but in the last century the term amplifier was used to distinguish one from the other.
If it is an “amplifier” it can pass through enough signal for the system to be audible even though the amplifier is not working. I’ve seen this happen with small amps/equalizers that were popular in the 1970s and 1980s, as well as some amps that were made by Lear Jet and Craig.
If so, you should be able to troubleshoot by unplugging the amp and plugging a small 8ohm speaker into one of the outputs coming from the main unit. If you get reasonable output levels, the amp is the culprit.
