Primary brake lights not turning on/getting brighter

The rear bulbs have two filaments, one is the marker/taillight filament. It is the one that comes on with the headlight switch. The brake filament is a different piece inside the bulb.
While it is unusual, both bulb brake filaments may have blown at the same time. I would try two new bulbs first. Or if you have a test light handy you could unplug the connectors at the lights and gently touch the probe lead to pin one (white wire with a tan trace) step on the brake, if the test light comes on you have power to the light housing, then plug them back in and try new bulbs.
If however you test them and find no power. Those wires go to a splice point and then into the harness under the vehicle. You would need to follow the harness and see if there is any damage or breaks in that wire (white wire tan trace).

Because the other lights work in the same housing the ground should be good so the issue should be on the power side. The problem there, if new bulbs do not change things it could be a bad integrated power module. The upper light is fed directly from the brake switch. The lower lights are fed through the switch into the TIPM under the hood. Then power goes to the rear. Has this vehicle ever had a trailer hitch installed? If yes is it factory or aftermarket?
I ask because if someone just tapped into the system without using the factory trailer harness it could place a higher load on the relay in the TIPM and burn the contacts. Then it is just a matter of time before the lights would fail.
If no hitch then this is not likely the issue.