Through the fiberglass bottom fairing
I have electric elevator and aileron trim, both wire bundles just have flexible sheathing over them and pass through simple grommets . If I were doing rudder trim, I'd run mine down through the rudder into lower fairing and into the fuselage with same wire I have going to tail beacon. A multi wire plug would facilitate rudder install, but add weight/failure point. You could just terminate splice at servo and leave service loop. If you send a message to Pat Hatch (On here), he has a very nice rudder trim on his -6 he may share details about. At the point of assembly you are in, you may want to entertain an integrated tab similar to the elevator tab for a better look over all. I did that with my aileron trim tab, and glad I did. A bit harder to make/install, but the end result is nicer. I don't know if the zipper T/E is an issue though.
8A7 / Advance NC