and if the fuse is good...
It might be the directional switch,if everything else is good..I practically rewired a truck's lighting system once, from the cowl back because only one brakelight would light up all the time,and one either would not light,or only worked when it felt like it--sometimes neither one worked!--drove me batty for a week!
..I cleaned all the grounds,checked bulbs and sockets..and finally I found it by accident--I bumped up against the directional switch lever as I was stepping on the brakes from outside the cab with one foot,and I saw the light's refection on the garage door!..
If I held tension on the switch lever,the brake lights worked every time!..and if I let go,it was back to only one most of the time..the contacts in the switch were they mount in bakellite plastic were loose,and the plastic was melted enough to let them make poor contact..I remember it being pretty expensive for a new switch,and not much fun to put it in!..it was a ford truck with a ramp body(1970),but I've seen GM trucks with the same kind of symptoms and they also had faulty directional switches..
Make sure the wires to the stoplight switch are not chafed anywhere or grounding out..and all the plugs are tight and not green with corrosion...see that 12V is present at the switch wires with a test lamp..
