Many years ago, radios had a rotating turret full of crystals of various frequencies, and a big knob with gears behind it with which this was rotated between a set of contacts.
All modern radios use digital frequency synthesis, where you (crudely speaking) take a crystal (just one crystal), and multiply the frequency using a phase locked loop within which is a programmable counter. So you have various other frequencies floating about inside the equipment, and their harmonics.
Plus you have all kinds of other muck e.g. a switching power supply running at some tens of kHz and loads of harmonics coming off that.
I can't believe Garmin have done this badly because they sell thousands of these things, but maybe there is a specific installation procedure (earthing specific things) which has not been followed? My experience of UK avionics shops is that universally they do not understand the workings of the equipment and merely follow wiring diagrams.
I would start by setting everything up to show the interference and then, behind the panel obviously, clip split ferrite beads onto one wire at a time (close to the respective equipment enclosure) and see if the problem changes. If you see any change then you've got it - it's conducted/radiated.