I just had the Alpine 9820XM installed but the subwoofer speakers behind the seats are not working. Any suggestions before I bring it back so I appear smarter than the installer although I question his brilliance since they don't work.

Sorry. I forgot the sound is cutting off every few seconds on only XM. Is this an antenna issue? He put it on the dash. Is there a better place? Is a glass mount better since it is higher up?