Trailer wiring harness

Sounds like its shorting somewhere, check the wiring.

 

First thing to get is a flat four tester (currently under $4 at Amazon with free S&H with prime membership).

See the following:

This way you can test whether the problem is with your harness or with your trailer.

If the problem is limited to your harness (and NOT the trailer wiring) then examine where you are picking your running light signal from. On my 2006 Prius, the wire colors found in the left rear of the car at the white electrical connector plug attached to the clip adjacent to the taillight/turn-signal assembly:

running lights= solid dark green* (found at left/driver side rear white connector -thick wires)

If you are using a running light signal location that is a different location than the solid dark green wire referenced above then try sourcing this signal from the solid dark green wire located on left side rear of the car adjacent to the taillight assembly (but accessed through the plastic cover located in that area.

*- other side of this same connector has the same running light signal carried by a brown wire (at left/driver's side white connector with very thin conductors/wires). Thin wires are so thin that I am reluctant to tap into them.