Was the tuner installed with the Mk III, and if so did it work then? If it did then it's possible that you have disconnected the anttena from either the tuner or the amplifier.

the tuner worked fine with mk3..this problem is only with mk4..i don't have the oem antenna or amplifier for tuner but only a little wire builted by myself..anywhere..this is not a big problem for me..i have fond the trick to make the switch work fine (go to av channel before return to gps menu)..