I also installed a flasher module and LED light bulb on the third brake light on my '86 Cab. I installed the module in line on the wire running under the rear spoiler to the brake light. You need to use an LED light bulb since the incandescent bulb reacts too slow to the flasher. Since I installed the flasher last year I have noticed other drivers leaving a little more space behind me and braking earlier.
Juergen