Probably didn't notice when I bought it, so it likely came this way to me. All lights work fine with exception of the tail light. I've replaced the bulb, but found nothing wrong with the one it had. It lights bright for either brake but isn't illuminated when the key is on. Took a look at the connection under the seat and found this. I think black would be ground. Red and Yellow brake? What's odd is that the harness going into the fender and back to the light only uses two wires. It ignores the center (blue) wire. Does this look stock or was something changed back before it was sold to me? I do know the seller put a new battery in. Besides a DayMaker front headlight, the bike looks stock. 2016 Sportster SuperLow 1200T.

 

Does it use a two filament bulb, or just one filament?

 

It's a two filament bulb. Sylvania 3157. Found the wiring diagram, but this is not my strong suit. Looks like only the brake and ground are supposed to go back to the tail light. Turn signals work, so those four wires (two separate connectors) are good. Can anyone read this diagram and tell me how the tail light gets power when the ignition is turned on? IMO can't be coming from the brake harness. Are the two turn signal harnesses powering it? Maybe the wires labelled BE? Still looking for my multimeter. My son used it over the summer and he's back to college now...

 

Under the seat is three connectors that takes power and signals to the back of the mudguard. That is connector 40 and it does only have 2 wires. That is correct.

Behind the rear lamp is a board with the connectors for the lights and indicators.

There is a 4 way connector that goes to the rear lamp and depending on Domestic or HDI pin 1 or 2 has the blue wire for the running lamp.

Wiring diagrams here:- 99949-16_en - 2016 Wiring Diagrams | Harley-Davidson SIP

 

Does the Sportster have a fuse box? I see the main fuse block. Any other fuses? Pulled the battery to look around and don't see anything besides the block. At least I'm getting to know the bike.

 

For some unknown reason I am not seeing your post here in the thread.

The answer is it has 3 fuses, maxi, battery and P&A.

All of the circuits on the bike to do with lighting are protected by the BCM. It can detect open circuits and over current situations and will raise a code if a bulb doesn't work or the wiring goes short circuit.

 

Tested everything with a multimeter and here's what I found. This is the harness that holds the light bulb. 6837003 is the label.



I have power with ignition going to wire #2, but this harness uses wire #1 which is not hot. As stated before the brake works, so those wires are correct.

The part number seems to match with what I see for a sportster tail light, so maybe wire #1 should also be hot with ignition but isn't for some reason..

 

The pigtail looks like there is only one blue wire. The wiring diagram looks like there should be 2 blue wires. I do not understand why.
Edit: Reread the posts "depending on Domestic or HDI"

A fix for you may be to pull the pin with the wire out of the pigtail connector and move it to position 2 instead of position 1. Why it is the way it is right now is the mystery to me? If someone had to pull the pins for a wire routing job (like a rear turn signal relocation), that would be an easy answer that they put the pin in the wrong hole.

The shop manual should tell you how to undo the catch to remove the pin.

Just a thought, hope this helps,
Tom