Not sure I can help with a solution but I have a few questions.

What radio did you install?

Does it have its own built in amp or is it a preamp unit?

Are you wired from the radio to the MB amp in the trunk then to the speakers?

I have always replaced the amp when I replace the radio. So not sure if an aftermarket radio will work with the MB amp.

Have you checked the wiring for continuity with a circuit tester?

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91 350SDL - my DD
87 190D 2.5 - son's DD
00 E320 (wife will no longer drive a diesel)
87 300TD - project?
91 190E 2.6 - parts
91 350SDL - parts