Aftermarket Car Stereo/XM Install Issue

[rabbitdog]

Hoping someone has run across this before, because the last thing I am is a car audio guru...

Recently I got a new sony stereo for my 04 tdi new beetle, and had it installed by a "professional" along with an ipod hookup for in the glove compartment.

A few days after the install, some of the audio sources (cd, xm, and ipod) just vanished (after starting the car) and the only thing I was able to use was the radio.

Took it back, they tinkered around, and it I got it back working.

A few weeks later, same thing.

I'll shorten a VERY long story down to today - after a bunch more reinstalls/tweaks, trips back to the installer, all my sources stay put, EXCEPT my XM. (They had even tried different sony stereos)

I even asked my NB dealer about it, but they hadn't heard of that happening with an aftermarket stereo.

Interestingly enough, my XM came back (briefly) after my battery died a few weeks ago and I had to get a new one installed.

So I'm thinking it some VW electrical thing?

What don't I (and the "professional" installer) know that would fix this in about 10 seconds flat?

Thanks in advance...