Just tighten-down the squelch on the VHF rig a bit. All electronics generate RF noise. All. You can pick up some ferrite donuts at certain electonics supply places or Ham Radio swap-and-shops, loop your radio and GPS power through those a few times and reduce somewhat any RF noise that's leaving/entering through the power leads. (That'll also somewhat reduce engine alternator noise in the radio.) That may help some. Other than that: Just make sure your grounding is good, and your coax in good shape and well-terminated at both ends. That's about all you can do.