Guys, if the left tail light is working, it ain't in the tow vehicle and it ain't in the 7 pin. Both of the tail lights work off the same wire from the 7 pin. If one is working, the tow vehicle, the connector and the wire from it is working.
OP, most of those fixtures have only 1 bulb. It has 2 filaments. It has a dim filament that works with the tail lights. It has a bright filament that works with the brake lights, and also with the turn signals. That is what they are calling a single bulb fixture. It usually takes an 1157 bulb.
Note the 2 filaments in this 1157 bulb's photo.
This is another angle of the same bulb. Note that there are 2 contacts on the base of the bulb. Each filament is connected to 1 of those contacts. Both of the filaments get their ground connection through the brass case forming the base.
Since we know the left tail light is working, you are getting current from the 7 pin. Now the question is why is the right side not working? If that was the only problem, there would be several possible causes, but there is another problem. The right brake light/turn signal is also not working. Since the odds are that you don't have 2 separate problems, we need to find a single problem that will cause both of those filaments to not come on. The only common point those 2 filaments have is the ground.
The choices are:
1. both filaments burned out. unlikely-but possible, need new bulb
2. bad connection between bulb and fixture-more likely, but still not my first suspicion, remove bulb and lightly sand contacts of fixture and bulb then re-insert
3. bad ground connection between the fixture and the frame-most likely scenario, by far. Attach new ground wire from fixture to frame.
OP, most of those fixtures have only 1 bulb. It has 2 filaments. It has a dim filament that works with the tail lights. It has a bright filament that works with the brake lights, and also with the turn signals. That is what they are calling a single bulb fixture. It usually takes an 1157 bulb.
Note the 2 filaments in this 1157 bulb's photo.
This is another angle of the same bulb. Note that there are 2 contacts on the base of the bulb. Each filament is connected to 1 of those contacts. Both of the filaments get their ground connection through the brass case forming the base.
Since we know the left tail light is working, you are getting current from the 7 pin. Now the question is why is the right side not working? If that was the only problem, there would be several possible causes, but there is another problem. The right brake light/turn signal is also not working. Since the odds are that you don't have 2 separate problems, we need to find a single problem that will cause both of those filaments to not come on. The only common point those 2 filaments have is the ground.
The choices are:
1. both filaments burned out. unlikely-but possible, need new bulb
2. bad connection between bulb and fixture-more likely, but still not my first suspicion, remove bulb and lightly sand contacts of fixture and bulb then re-insert
3. bad ground connection between the fixture and the frame-most likely scenario, by far. Attach new ground wire from fixture to frame.
