Rear brake/turn signals not working

I have vehicle listed above that has had an ongoing issue with the rear turn/brake lights. The tail lights do work in normal running mode, turning on with the headlights. However, neither the turn signal, nor the brake light comes on when triggered on either side.

The front turn signals do flash as normal when using the turn signal switch and the hazard lights.

I have replaced (more than once) the bulbs on both sides, as well as replaced fuse 12, which is indicated as the turn signal/brake signal fuse in the manual.

In the past, I've seen the driver's side briefly fail to work for two to three days, before starting to work again for no apparent reason. It would happen, and then things would be fine for months. However, most recently the driver's side went out and stayed out for three days, and then the passenger's side also went out; neither side has started working again since (two weeks, at this point).

I have a suspicion that the issue might be wiring related, as at least a couple of times smacking the panel over the driver's rear wheel seemed to stop the issue, and would love to find some sort of guide or diagram for getting in there to inspect the wiring to see if I can locate a break, but any advice would be welcome.