Mine is a -7, but the rudder is the same, so I'll throw my solution out there anyway. Unfortunately I didn't take a lot of pictures when developing this solution, however I think you will get the drift. I put way too much thought time into this (a common problem and the reason it has taken me almost double what it should to build this thing) however I think the solution meets my mission requirements.
I didn't make the fairing removable, as I didn't want to add weight aft, since CoG is quite critical in many -7 builds. The tail light cable (22 awg shielded two pair) is in a piece of conduit that runs the length of the tail and is supported by (and runs through holes in) two balsa ribs (I paint them with epoxy which makes them strong, light and waterproof). The contuit was sized to be just large enough to hold the mini molex plug I was using. In hindsight I think that I should have just used a gold JST-SH RC plug as the current usage of these LED tail lights is so low (another happy Flyleds.com customer). The conduit curves upwards at the forward end and is covered by an aluminium face plate. This plate is removable. It's held in place with two NAS1801 #8 screws (a favourite of mine) into nutplates, so that if I ever have to remove the rudder, I can undo the faceplate and pull the cable through completely out of the conduit. The cable curves in a 180 degree loop so that rudder motion results in twisting rather than bending, reducing fatigue loading. The cable at this point is covered covered in heatshrink for additional wear protection. It is zip tied back to a bracket that is mounted to the aft bulkhead. This bracket is in turn bolted in place when I mounted the vertical stabilizer. The cable then goes through a 1/4" hole in the bulkhead. IIRC there are two snap bushings in this hole (one pushed from the outside and one from the inside), as the stack thickness was so great that one bushing didn't give me adequate protection on the inside. Just forward of the rear bulkhead, I have a service loop long enough so that I can cut the zip tie and pull the wire aft and disconnect the tail light.
There are of course drain holes drilled at the low point in the conduit and the low point of the rudder fairings, however with the faceplate on, I suspect there will be little water or debris entering the conduit.
Hope this helps.