Try this:
http://www.sacskyranch.com/degauss.htm

Jim wrote:
The wet compass in my Cessna 150 has suddenly decided to indicate way off.
I took it off the dash mount and it reads nearly correctly when moved about
12" away from the instrument panel, and even better when out in the open by
itself. I got another wet compass from a friend, and as I moved it near the
instrument panel got the same effects. It's at least 40 degrees off and in
some places like when held over the firewall its over 90 degress off.
Nothing else has been done to the plane recently. Is is possible that
something in the plane that has become magnetized and is affecting the
compass ? Is there anything that can be done ? I pulled the nav-com out of
the panel and it didn't seem to make any difference. All this is happening
when the master switch is on or off, doesn't make any difference.