Dynon EDC-10A Mounting location
I tried the same type of installation using a fabricated tray for the back of the EFIS-D10A. With power on and normal systems activated, the compass still gave acceptable indications (within 5 degrees of mag compass). However, when I transmitted on either com radio, had 40 degree heading swings. I ended up mounting in a location islolated from all significant load carrying electrical wires by at least 12 inches and the heading information is rock solid and spot-on with the mag compass all the time regardless of electrical load, on/off, transmisions, etc.
I used a Sears digital level to check the alignment in pitch and roll with the EFIS-D10A and got it within 0.3 degrees in roll and 0.2 degrees in pitch. Yaw alignment does not seem to be as critical as roll/pitch since you have the ability to correct that in the EFIS to EDC calibration procedure.
Reggie