third brake light on 1989 dodge ram 1500

my shifter does not work at only goes into drive it's not a shifter is automatic but when I drop put it in reverse it goes with Ford if I put into a Dro and go forward go put it in drive it goes board so I don't know what to do thank you

I'm guessing the third brake light would run off the same circuit as the brake light. So if the third brake light is "out", test for voltage there and you might only have to replace the bulb, not a fuse.