A friend with an E-Racer had a hopeless panel-mount compass error.  The
problem was finally traced to the wire bundle passing behind the compass.  He
sometimes overdid shields (grounded at both ends), underdid shields (none)
or, perhaps created ground loops (she ain't no taildragger).  He finally
moved his 12th Century Technology compass to the far side of the panel and
mainly solved his problem.  Note that the E-Racer's engine is in the rear
with the alternator as far away from the panel as possible (the pilot is also
forward facing).  

Personally, I use a Vertical Card Compass (a joker for the FAA) mounted on
top of and at the center of the glare shield.  It is only off V to XIV
degrees (12th Century math to match) whenever I choose to look at it.  I
mainly rely on an electronic compass and GPS ground track.

Note: Luckily I live in the Chicago area (0 degrees correction) and
appreciate True North representation - The rest is guess-work when I travel
to other longitudes and ATC only expects you to be within 10 or so degrees.  
If you use GPS, just ask ATC for the ground track they would like you to make
and then surprise them by actually flying that track.

Grayhawk