[JLMounce]

It's going to be a poor or non-existent ground somewhere. All of the lights up front are grounded to the same circuits.

If you're chopping into wires, re-arranging a bunch of stuff and fighting this, I'd highly recommend a new forward light harness. They are readily available, plug and play and aren't too expensive. If you've got enough going wrong there with the lights, I'd do the engine harness at the same time as they both exit the bulkhead connector.

Like you I fought lighting wiring for a long time. Putting new wiring in the car was one of the best things I did from a reliability and ease of use standpoint.

Edit: I should also point out that the bulbs themselves are part of the ground circuit. The bulbs ground to the contacts in the sockets and the sockets will also typically ground to the lenses (this may not be the case for the markers). If the bulbs or sockets aren't grounding properly, that will also cause the issues you're seeing.