Matt was able to track down another nutplate for the Garmin GMA340 audio panel harnesses from his avionics guy, Dennis up in Rochester. We purchased our two audio panels from Dennis and he kept the installation kits since both Matt and I are using the Approach Systems pre-made harnesses. Unfortunately we still needed the special nutplates that are used to attach the D-Sub connectors to the backplate. I'm glad Dennis gave these back to us because I couldn't find any place on the net that sold these separate from the installation kits, which are way too expensive to purchase just for a couple of nutplates.
Once the J1 and J2 Approach Systems harnesses were screwed to the backplate I screwed the backplate onto the GMA340 tray and slid in the audio panel.
Garmin GMA340 audio panel harness nutplate
Garmin GMA340 audio panel with J1 & J2 harnesses screwed to backplate and backplate installed on tray
