The AGC (signal strength) and the MPX signal are on separate wires, therefore you could have full scale deflection on the signal strength display but no audio.

Yup. There's a FAQ entry specifically on this. Although that FAQ entry describes the factory supplied tuner module and extension cable, perhaps it's a similar problem. Check the pinouts on your connector and double check the wires themselves and how they're soldered to the module's circuit board.

Edit: I see that you say you've treble-checked this stuff already. Ah well, then, I don't know what to tell you.