[DrDave]

Well, at least you're not idle on the project. The avionics master simply splits the bus and puts a switch in the middle. Rarely does this have any issues. The picture below shows the setup I use for bypassing panel radios and power. You can pull the coax down and plug right into the radio. It's a nice way of testing the antenna system. It's certainly not like a VSWR meter but it will give you a functional test and a chance to hear any antenna interference. The radio is an ICOM IC-A20, an antique by today's standards. Other airplanes will not be able to tell you are on a handheld versus a panel mount 10 watt radio. Use the handheld radio and then hit the 1/2 split on the Garmin audio panel and see what that does.

I think it would be fun to wire an independent battery to power your radio. The options will be narrowed down after all of this.

As to how avionics guys keep their sanity: "these old radios never work," "you really need to upgrade everything," blah blah blah.

This can be fixed.

Dave