My right side rear turn signal and brake light haven’t been working. I changed the bulbs, I changed the flasher relay since the front signal also stopped flashing, I checked the sockets and I was definitely getting power at them. I took the housing out and my mechanic gave it power with a power supply, and it worked. Our best guess was a bad ground. I cut my ground wire, grounded it somewhere else, no luck.
Then I came across a thread I missed on the Montero Facebook group that mentioned the hazard switch is known to cause turn signal and brake light issues. I drowned it in contact cleaner, and voila, I have all my turn signals and brake lights again.
I really wish I’d tried the hazard switch sooner, but it just didn’t occur to me at all. Oh well. It all works now.