How to test radio system
 

The radio amp in my 190E seems to be bad.Tthe PO bypassed the amp by cutting most of the wires so I fixed that mess and now only the dash speaker on the driver's side works. The car has the Alpine 6 speaker system with active loudspeakers and I figured that something was wrong with my radio because the dash speakers were getting overloaded with bass though the amp is supposed to filter it to the door speakers. I discovered the hackjob when I was tracking a fuel leak. I used butt connectors install a new pigtail.

This confuses me. I though the amp was only for the door speakers yet all of my other speakers except one has stopped working. The wiring diagram is 124/4 in the electrical troubleshooting manual. http://www.startekinfo.com/StarTek/outside/11832/Resources/201Create/PDF/01002.pdf

Anyone have ideas as to what went wrong and how i can test the system? I am going to check the fader but I believe that has been bypassed.

I found that my fader was disconnected completely. I went ahead an bypassed it, the brought in the rear speakers. The passenger side dash speaker was simply a loose connector. Still no door speakers. 12V goes to the amp.