Can’t figure what’s the deal with the relationship between the brake sensor and the different brightness and how to fix?

 

What year and model?

 

2011 s 80 3.2

 

Not sure what your question or problem is. What do you consider the "brake sensor" to be?
The brake lights will be the same brightness on both sides. If not someone may have installed the wrong bulb. The car does have a rear fog light on the left side - that is as bright as a brake light, and confuses some owners when switched on. Switch for it is next to the light switch (and front fog light switch) The parts catalog show 4 replaceable 21 watt bulbs in each tail lamp - yellow ts bulb, clear reverse, brake and rear fog. The 21 watt bulb is a standard 1156 single filament bulb either clear or yellow.

If you invoke ABS or "brake suddenly" - "more brake lights" come on as described below. I would imagine that it turns on both the rear fog lights in addition to the normal brake lights under those conditions. Don't know what it does with the LED parking lights - Does it brighten those also? -

 

It’s the LED lights on the top of the housing for the tail light one is really bright and one is really really dim

 

I would first scan all the control units with a compatible scanner - looking for codes/lighting abnormalities in the CEM. You can also turn those lights on individually with a good scanner. Water intrusion into the CEM seems to be a common theme - that can do all sorts of crazy stuff, but usually more symptoms than just a parking light that's too dim or bright.

If nothing abnormal unfortunately you may have a tail lamp going bad. The LEDs are not shown as being sold by themselves. To prove it and make sure it's not the wrong signal being sent to one side - if the connectors are the same on both lamps - I would take both tail lamps out and plug them into the opposite side and see if the dim/bright follows the lamp. .