01' A6 Quatro

I'm having reception problems, I want to check my antenna connection. Where is it located?

 

Around the rear window, its probably not the antenna though.
First remove the radio and try puting a multimeter to the port on the back of the radio for the antenna (black lead) and see if the reception gets better. If it does the radio is ok.
Second check the connections at the antenna but its more than likely the antenna amplifre.
Pop off the trim on the center of the headliner in the rear, there should be several modules bolted to the roof, look for the one with radio cables hooked to it, try wiggling the wires to it or un-bolt it and move it around, if the audio quality changes thats what the problem is.

 

I thought the antenna connections are in the driver side D-pillar along the side of the rear window. Here is one connection back there (picture from my power rear sunshade project, just above the side curtain airbag):