Some questions: Does anything change if you push on the brake pedal? Does anything change if you separately turn on park and head lights? Do the tail lamps seem alright- bright, dim? Do the tail lamps change brightness when the brake lights flash? What changes if you remove the flasher- lights go out, lights remain on, brake lights work normally with the brake light switch? My first thought is that something in the rear light ground circuit is involved with your trouble. If the rear lights are dim, even though they are flashing, I would start checking ground connections. I don't have a wiring diagram to look at, but if the rear lamp circuit is getting grounded through the flasher/front end ground, you could get that kind of operation. This is speculation at this point. If you have a voltmeter, rig up some long leads so you can measure voltage between the battery ground and the rear lamp ground. You should see only a tenth or two voltage on the meter with the lamps on. Connect the voltemter with the negative probe at the battery ground, positive probe at the lamp ground, so the meter will read up-scale. If the reading goes negative, switch the connections.
As a shortcut test, connect a jumper wire from the tail lamp housing or lamp socket to the battery ground. If the lights work normally, you have a body ground matter to resolve.
The switch is a possibility, but I would first look at the circuits.
Harvester
