If you are going for the advanced avionics that most of us seem to be using, anything with a magnetometer, don't use the airframe ground (that's for a position light anyway, not a strobe). I made up a cable for the tail light and my rudder trim and let it drop into the bottom fairing (caterpillar grommet on the fairing edge to prevent chafing) to a CPC attached horizontally to the rudder root rib. Don't let it dangle and remember to put a drain hole in the bottom of the fairing. Yes, my -10 isn't flying but my -6A has a similar, if simpler, setup for the tail position light.
Patrick Kelley - Flagstaff, AZ
RV-6A N156PK - Flying too much to paint
RV-10 14MX(reserved) - Fuselage on gear
EAA Technical Counselor #5357