Q: My tail-brake lights are not working, but my top brake light is working, and the taillights are on when the headlights are on

I recently discovered that my rear tail-brake lights are not brightening when I brake. My top brake light is illuminating when I brake, however. Also, the rear tail-brake lights are on when I turn on the headlight. Turn signals work fine as well.

I've been looking into possible problems (brake light switch, fuses, etc.) and saw in the factory diagram that there is a relay box for my model that contain 4 relays: a "taillight relay," a shift lock relay, a seat heater relay, and a high beam cut relay. The diagram was mildly confusing and it was hard to tell which relay was which. However, I noticed that one of the 4 relays was empty (there was no relay). It seemed to me that the missing relay might in fact be the taillight relay.

As I explained earlier, my taillights still burn...is this even possible without a "taillight relay?" Also, can I test my theory by moving one of the nearby relays into this relay, or are these relays different (like the different amp. for fuses?).

My car has 188000 miles.
My car has an automatic transmission.