Replace the flasher relay.
Can also be a weak ground connection at the RR connection. Good Luck as these are sometimes very difficult to find.
Probably Flasher
try replacing the flasher. there should be two, one for flashers and one for turn signals
Check grounds in light sockets Check for corrosion and loose connections
check the brakelight switch. it is located under the dash where the brake pedal rests. Hey its ritchie. If you can click your blinker down and then they work its you blinker switch,but this is my monte.
It has been my experience that when the blinker comes on but the lights don't blink there is one or more bulbs burned out. If that is not the case check the flasher.
A simple first step would be to check the fuses to these lights, then go from there.
Sounds like it could be a bad ground
The next thing to check is the switch that activates the brake lights. It is probably on the master cylinder.
The tail lights are on a separate wiring than the blinker/brake lights. Look for a blown fuse or a broken/loose wire.
cheack the wireing
your problem could be in the switch its self. unfortunetly they are expensive to replace, i had to actually wire a switch direct to the blinker system.
dome lights do not work. checked the fuse which is ok. checked the bulbs which are also ok. where is the problem