I always had 1 dimmer on my duster, I took the dim tail light apart and the reflective coating on the in side of the bezel was gone. I sprayed it with chrome paint and now its bright. Don't know if this is your same problem, but it's worth a try.
Brake light switch stuck on... this would be where i check 1st .. On my old val the brackit bent just enough to let them stay on took me alittle bit to figer it out, If the car aint running there going to be dimer then if it was running