Hmm. I'm in uncannily similar situation. Signal strength at full, but the reception is terrible, and very quiet.

Can anyone offer any hints on how to debug this problem? What do the different wires on the tuner connector actually do? Is there a schematic for the circuit, so I can start narrowing down where to look? I have access to an oscilloscope - are there any points on the board to check for a particular signal?

I've also got a MkIV Golf (yes, I've got the amplifier), but it doesn't work on the test bench either :-(