You took good care of it if it has that many miles. As far as the light, there is no dead end. I always have other ideas. LOL
Now, I went through the schematics again. For power to reach the taillight (RR) it has to go through the park light relay. However, if the relay was bad, you would have both rears staying on. So, that isn't the issue.
That (at least in my tiny mind) means we have power between the relay and the bulb. All parking lights are powered via a brown wire for the left side. There is another thing I want you to inspect. (If you haven't already). On the left rear of the frame rail is the tail lamp junction box. Locate it and inspect the wiring to and from it. Confirm the connectors are not damaged or have any obvious damage. Disconnect it and see if the light turns off.
I attached a pic of the junction block below, so you know what to look for. Never lose faith. We'll figure it out. Wiring usually requires simply narrowing things down.
See pic below.
Saturday, March 12th, 2022 AT 4:53 PM