2003 Passat Remote Locking Problem

My remote locking/unlocking has been getting "weaker" and "weaker" i.e has to be operated from closer and closer to the vehicle to the point that it is almost no longer a remote. I have changed battery in both keys - seemed the obvious solution - with batteries from different sources but to no avail.

Any ideas?

Where is the receiver module? Perhaps something is wrong there although the dealer says when they go faulty they just stop working altogether.