The biggest culprit would be the ram microphone. The second biggest concern would be the power connections to the Garmin and the ram microphone.

The microphone has a electro magnet that you want to keep away from the compass -- certainly a good 12 to 18" at least. Edson makes some pod attachments -- are you planning on using one of those, or as Paul suggests, something attached through a pedestal or binnacle guard?

In any event, I'd stay away from the Ritchie compass with anything powerful.