Update:

Looking through the 396's Setup menu, I found a field called "System
Mode" which was set to "Battery Saver." When I set this to "Normal,"
all the problems associated with the XM antenna went away. I don't know
if I put the thing in Battery Saver mode or if it did it itself, but
apparently letting the battery run down while in this mode is a no-no.

Damned freakin' gadgets.

--
Dan
C172RG at BFM