Still having trailer light issues

On a 5 wire plug one can be used for trailer brakes. When backing up it will lock out the surge brake from functioning.

 

I agree

 

This is MarkB's other thread on trailer lights. 5 wire flat prong trailer wires into a 4 wire https://www.boatdesign.net/threads/5-wire-flat-prong-trailer-wires-into-a-4-wire.59269/ It is a Flat 4 connector with 5 wires for a wishbone harness, not a Flat 5 connector with a backup wire. The only photo in the thread shows a connector with five wires but does not show the number of connections. There are 1 green wire, 1 yellow wire, 2 brown wires, and 1 white wire. Note that the brown wires are next to each other, while there is a space between the other wires. The harness has one brown wire for tail/maker paired with the green wire for turn/brake on the right side of the trailer, and one brown wire for tail/marker paired with the yellow for turn/brake on for the left side of the trailer. Link to a vendor's photo of a similar harness.
20 ft. Wishbone 4-Way Trailer Wiring Harness with 30" Ground Wire Optronics Wiring A20WB https://www.etrailer.com/Wiring/Optronics/A20WB.html?feed=npn&gclid=Cj0KCQjw95vPBRDVARIsAKvPd3I0M0htLbDqNXs3pAg8tixLUXIl4YfM0c-cYVdG9OpmJ4EXYAdG4B8aAkrAEALw_wcB

The tow vehicle connector needs to wired the same as for any Flat 4 connector.

 

Not for nothing, but any reasonably skilled mechanic could sort through your wiring issues, in 5 minutes or less. You seem to be clearly out of your comfort zone, so take it down to a local automotive shop and have them put a 1/2 hour of effort into this issue. If you brought it to me, I'd know what was wrong in less than 5 minutes and could execute a fix in less than 10, assuming the car's wiring was reasonably available. Most of this time would be spent crimping wires into appropriate locations. It may be as simple as needing a converter box, for those cars that use separate bulbs, so they can use the same for multiple functions. There are three of four types of these boxes, running from $10 - $20 bucks each (you'd only need one). Given the amount of time you've spent, maybe a wiser path to follow . . .

 

The original poster said the lights on a different trailer work okay with the tow vehicle, which means the tow vehicle is very unlikely to be the cause. The problem reported affects both trailer lights symmetrically, which implies to be that both trailer lights have the same problem. I had a very similar or identical problem to the one reported and it was due to the lack of a ground connection between the trailer lights and the tow vehicle.

 

I agree, it sounds like a ground, though a length of wire marrying the vehicle and the trailer harness, will quickly prove or disprove this concern and takes a few minutes to apply. Hell, a set of battery jumper cables; one on the car's frame and the other on the ground termination on the trailer, will very quickly provide an indication.

 

https://www.amazon.com/CURT-57505-5...8592321&sr=8-8&keywords=trailer lights tester

 

Why a 5-way tester when Mark B, the original poster and the person with the problem, has a 4-way connector with a wishbone harness?

 

Because it tests power at every terminal

 

But there are only 4 terminals.

 

Not on the photo he posted. There are 4 male connectors and 1 female for ground.

 

I see a 4 connector plug in that image, not 5.

 

Are we talking about the photo in the other post? Either way, a 5 connector tester works fine for a flat 4. One connection is left out.

 

Agreed, but you do need a bit of a clue as to what you're doing and if you do, you don't need to buy a tester.

 

Yes, the which shows 5 wires going to 4 terminals on the connector.