I had a similar problem on my BMA, although the interference was coming from the WigWag system on my landing lights. Every time the WigWag flashed, the compass swung about 5 degrees. The magnetometer was mounted in the wingtip, about 2 feet from the landing light, but the wires all ran in the same conduit and passed within a couple inches of the magnetometer. Shielding the landing light cable didn't work and I couldn't get enough separation to make everything work, in the wing tip, although distance from the light wire did help. (I did not try twisting the power and ground wires to the landing light, since I was grounding to the wing.)
I finally relocated the magnetometer behind the rear baggage compartment, on my RV8. Just built a small shelf on two angles spanning the two upper longerons.
I don't know how your magnetometer is set up, but if it has a connector, just make a new connecting cable and run it to a new location, away from your interference sources. (Such as the radios, stobe power sources, etc.)
You could make up a long test connector cable and try out possible locations until you get a good one and then mount it permanently. The interfence is registered by the magnetometer, not by the wires connecting it to the EFIS, so you should be able to run them just about anywhere, but using a shielded cable probably wouldn't hurt.
Anyway, that's what worked for me.