Check the 8-way tuner connector wiring, and sled wiring too (I think this is in the FAQ)
Yeah, the entry is here. It describes the problem only as related to the factory tuner, though. Still, worth double-checking.

Although in the case described in the FAQ, the problem is that the signal strenth meter is bright instead of dim yet the reception is bad.

I still remember clearly the day you guys discovered the root of that particular problem, standing in the parking lot in Santa Clara with Zandr, poking and prodding at his M3 installation. That was a fun day.
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Tony Fabris