I've got my GDL-39 on my dash with its rubber ducky antenna. A friend has his under his dash with a cable to a standard blade UAT antenna on his belly. When we're flying together he consistently sees more traffic than I do. If you're going to remote mount I'd use a regular UAT antenna rather than trying to reuse the rubber ducky. I'm currently installing an ADBS OUT/IN unit which will obviate my need for the GDL otherwise I'd switch to a belly mount UAT like my friend.
1950 Navion - flying
RV-6 - AWC received 2/9/2021
1940 Rearwin Cloudster project next
4 L-2 projects on deck