My D3 (2006 A8) gives me some intermittent errors for the access start antenna.

The unlock (touching underside of door handle works almost 100%. However, the "lock" button does not work 99% of the time in all the doors.

Is the lock button failure related to the access antenna or is it the button, perhaps a micro switch which is a PITA to reach/

 

The lock buttons are a common problem.
It is indeed a microswitch that comes loose on the board.
It is not user-serviceable, but some (Russians) have drilled it open and replaced with better quality components.