Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
It sounds like you need some ground loop isolators to filter out the signal interference..
Place one of those or similar on the positive power lead going into the VHF, and that will eliminate any signal contamination from the garmin.
If you have further qustions, let me know.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.