You want to test its location when it's "functioning" (there are some variation on some units depending on what they are doing.

If it's pretty self-contained the next issue is the power cord. If you're just using it with batteries, this isn't applicable; however, you should see if that affects the compass. Usually, if you run the cord inside the binicale guard supports (SS Tubes), you will avoid any issues.

I have a Furruno 12" unit above the compass; and, a Northstar 9" display just below it. There is some affect which we compensated for by reswinging the compass.

However, if the compass continually rotates or is fixed on the unit or... Then your unit really has a lot of EMF...