This is a perfect example of how repairs become exceedingly difficult and expensive due to a lack of structured troubleshooting.
While there are any number of reasons as to why your tail lights might not be working, you can't just randomly hope that one of them is the problem and that there is always going to be an instant fix. You'll waste more time and money that way. You could have bad tail light boards, yes. You could also have a bad brake switch. Hell, you might only have an improperly installed & configured brake switch because of incorrect tinkering due to a PO. It's uncertain. A misadjusted brake switch can cause this exact issue. The switch is always closed, and doesn't break it's connection until the button is actually pressed. It's wired opposite from what you normally expect from a button. So with your foot off the the brake pedal, the button is pressed in. Once you press the pedal down, it's arm lifts up off of the button to close the switch and turn the brake lights on. The switch is supposed to be tensioned properly so that as soon as you bump the pedal it makes contact. If someone screwed that switch all the way down, it will appear to cause an intermittent brake light issue. Pressing the brake pedal lightly while going slow doesn't force the pedal down far enough to turn the rear lights on. But pressing hard for quicker stops from faster speeds will result in the driver pressing the pedal harder, and will in turn then turn the brake lights on. Thus the issue appears to be intermittent in appearance.
But here is the best way to quickly test your brake light system. You're gonna need a multimeter. If you don't have one, get one. If you don't know how to use it, that's okay too. There are plenty of YouTube videos, or you can just post questions here. But I guarantee that if you follow this structured troubleshooting list below, you'll have your issue fixed in no time. This is how you should always go about repairing a problem instead of just shotgunning it each time.
Step 1 Using an separate, independent ground, check for the presence of 12V at the switch.
Is there Power (NO) -> >>>STOP<<< Check your fuses & repeat Step 1.
(YES) -> Proceed to Step 2.
Step 2 Check the switch for internal continuity.
Does the switch close as soon as you press on the pedal? (NO) -> Proceed to Step 3.
(YES) Proceed to step 4.
Step 3 Adjust the switch by backing it out and letting the button slide out until it closes. Then screw back in just past the point where the pedal pushes in to trip the switch and break the connection. Once you have adjusted the switch to close as soon as you press on the pedal, proceed to step 4.
*Note: If the switch entirely dead and doesn't respond at all-> >>>STOP<<< Replace Stop Lamp Microswitch (part #100820) and repeat Step 3.
Step 4 Disconnect the wires from the brake light switch, and insert a jumper in between to deliver a constant 12V to the rear brake lamps. Using an separate, independent ground, check for the presence of 12V within socket connections for the Rear Tail Light Circuit Boards.
Is there Power (NO) -> >>>STOP<<< Inspect wiring & bulkhead connections, and reseat/repair as needed. Once complete, repeat Step 4.
(YES) -> Proceed to Step 5.
Step 5 Check ground connections within socket connections for the Rear Tail Light Circuit Boards using the verified 12V source.
Is there Ground (NO) -> >>>STOP<<< Inspect wiring & bulkhead connections, and reseat/repair as needed. Once complete, repeat Step 5.
(YES) -> Proceed to Step 6.
Step 6 Remove Tail Light Circuit Boards, and test for continuity.
Is continuity verified? (NO) -> >>>STOP<<< Replace boards if original, or repair later versions.
(YES) -> Replace light bulbs.