240 Odd Brake Light Problem

[DeluxeRaccoon]

I have a 1990 240 DL sedan and have recently had some trouble with the brake lights.

When I press the brake pedal while my head and taillights are off, only two brake lights work, the CHMSL and the passenger side one. When I press the brake pedal when my head and taillights are on, only the CHMSL lights up and the passenger side brake light turns off completely. Either way, the driver side one doesn't become brighter as it should. I haven't tried to see what happens when I have the parking lights on, but I almost never use the parking lights anyways. All other lights seem to work fine. I have replaced the Bulb Failure Warning Sensor/Relay, all my fuses seem to be fine and are checked and replaced regularly, both taillight circuit boards are only a little over a year old and look to be in fine shape, although I tested a new one on the driver side and still have the same issue. I made sure the ground wires on the circuit boards are secure. I'm really confused here, I may just end up taking it to a shop because I don't like working with electrical stuff.

I've been looking at a lot of different forums and I can't seem to find anyone with exactly my same problem. I've seen a lot of forum posts where people have no brake lights at all, but my issue seems to be a bit different.

When I first purchased the car, the brake lights did not work and it turned out to be a mix of an unplugged connector behind the fusebox, and bad circuit boards. But everything seems to look fine to me.