The reception on my XM pretty much sucks. I'll get signal for about 15 seconds then go without for a minute or so.

I bought the car about three weeks ago with the XM already installed. The guy has the antenna mounted to the rear drivers side window. I'm thinking that my crappy reception must be due to the antenna placement. Does it have to be mounted to the roof or could it be something different all together?

...what kind of XM unit is this?

Typically, roof mount is the best...but SAT radio reception in each area is different.

You should test your antenna in a few places:
- roof
- rear deck
- front dash

and see what kind of results you get.

It's possible that the antenna wire is broken, which might explain your intermittent signal.

You can also search/post on the xm forum (xmfan.com).

Good luck!