I thought of doing this and called Garmin. They were OK with it but told me that the antenna is only tested to 180 degrees F. I do not know how hot the area next to the firewall gets. In flight probably not 180 degrees but after flight probably pretty close to that if not more. So I started asking myself if that heat would degrade the antenna over time. I don't know.
Sorry to be the potential doubt inducer here.
Maybe someone has run tests to see how hot the area gets after shutdown on a hot day.