Stephanie,
I would also check both the ground and positive cable / wire on the battery to make sure that neither is loose or corroded.

Did your car do this BEFORE you had the new XM radio installed? If not, call the installer immediately to let him know that you're having trouble with the car. Give them a chance to look it over and see if there's anything that they can do. You want to do that in case you need to have a repair made to the car and the mechanic shows that the repair was due to something that the installer did.

__________________
Jay Yambrovich
Scotts Valley, CA.

1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles

2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)