A member of our flying club sent this to me:

I looked into this when we bought the new GPS 396. While we can
remove the magnet from the base of the XM weather antenna, it will
void the warranty, which didn't seem like a good idea at the time.
Instead I made a steel plate that attaches to the dash by Velcro on
which to put the antenna. The steel works as a flux keeper and
contains the magnetic field enough that it has virtually no effect on
the compass. If you move the XM antenna and the steel plate together
from their current location to farther away from the compass you'll
see that the effect is almost, if not completely, imperceptible.

vince norris