[The Heater]

Your lighting runs off the same fuse, so unless you can find a mechanical break in wiring somewhere, your bulb in the tail lamp is bad. If all you did is low side the bike, I doubt you broke any wires. Try a new tail lamp.

You said the turn signals work, so I am using this information to answer your question. The circuit for the lighting is not by itself. You have a fuse for the headlamp that is a dedicated circuit. But the other lights are on a circuit that feeds other electrical loads.

 

[The Heater]

You state you have no voltage reading from the tail lamp socket. Did you check when you apply the brake? That is a three conductor socket. So make sure you have the test leads on the correct terminals or better yet put the test leads onto the terminals BEHIND the socket where they go in. You also can just follow the tail lamp wires back toward the Junction Box, and see if there are any connections through plugs, and test at the rear of each side of the plug set and see if any voltage is coming to that plug. Keep going back, and you may find the problem.