The only thing I can think of is like Jack mentioned on F150online about interference. Maybe try wrapping aluminum foil around the Edge cable to see if adding some shielding helps. I remember a person having trouble with their programmer because the cable was picking up EM interference from high voltage power lines. If it can receive significant amounts of interference it could possibly produce interference. If you have the cable coiled, uncoil it.
This is just an educated guess. I've never used a factory nav system so I don't have a clue of how sensitive they are. My GPS is a handheld trail unit that acts as an active following map. It isn't affected by anything but the cab roof. It also has full shielding inside the plastic case.
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