I've seen 2057 bulbs installed in older vehicles that should have 1157 bulbs ,and vice versa too,that can cause weird things--the bulbs look and fit the same,but the filaments are wired differently..
If you put a 2057 in place of an 1157,you'll have the brighter filament come on with the parking lights and the brake light will be the dim one!..
Sometimes a flasher will cause weird issues too,on older GM's you could swap the directional flasher for the 4 way flasher as a way to test that..
A friends ramp truck had the directional switch go sour ,so only one brake light worked most of the time--we spent days trying to diagnose the issue,and we would finally get both working,only to have it fail again minutes later..
(one day we got pulled over twice,by the same officer,and he didn't believe us when we told him we'd spent all afternoon working on the lights and re-wiring all the cobbed up mess someone had run to the rear of the truck--he gave us a warning the first time,the second time,he cited him !)..
We even tried jumping the right side brake light wire to the left side,figuring both HAD to light up then,but then when you went to use the directionals,both tail lights flashed!..
...
We hunted down a new directional switch,probably the last one in MA for a 1970 F350 at the time,only one old Ford dealership happened to have a dusty one sitting on the shelf...$58 + tax ,and that was around 1994!..
..got $100 worth of tickets for "defective equipment" too,in a week,the same cop waited for my friend to leave ,he parked down the street from his house 4 mornings in a row--threatened to have it towed the last time!....
