Yeah, if you look around this site, you will see nothing but pain from people who tried to add just nav to their MFT setup.
Your two options are to either get a nice mount for your Garmin or replace everything. I replaced pretty much everything in my setup.
That said, my suggestion would be to just mount your Garmin and enjoy it. Replacing everything for me was a significant amount of wiring and setup. I personally think it was worth it, but unless you want to spend a few days wiring everything and setting things up like I did, just mount your Garmin and enjoy driving :)