[LH Lead-Foot]

Can you provide some information on the vehicle year & model? This will help me to answer your wiring pains. I don't have any experience with aftermarket wiring harnesses as I wire everything my self.

Re-check the turn signal switch operation without the lights on. This will be a base-line test as the brake B+ passes thru it for several reasons. The passenger rear park light should be a simple test at the socket to a good ground. Double check the bulb to make sure it is indexed correctly and the right type, like 1157.

For the OEM rear harness, a common BRN is used on GM's for rear tail lights & license plate lamp. There is two wires at the body connector for the BRN tail light circuit, so check that also. DRK/GRN is right stop and YEL is left. LT GRN is backup lamps, so that your 4 at the connector. The tail light body is grounded thru the screws to the socket. I cleaned a dedicated spot and added a ground with a connector. Don't forget dielectric grease on the bulbs, terminals once you find the fix.

As you stated, with the head light switch on, everything works, including brakes, but no tail lights. High resistance at a bad crimp, terminal spread / loose in connector or bad ground on the rear harness for the tail lights. Plus, depending on the year, but there is a single bulk-head connector for the body and then one at the rear, left side. Also, depending on what you have, but if the fuse box has glass-tube fuses, check for loss of voltage at both ends of the fuse during the conditions that cause the loss of the rear lights. Hope this helps.