The Switch will have color coded wires and a diagram if you buy new, if used, post the model and we will help you google. Generally it's 7 wires as George says.
1. Fr Left lamp
2. Fr Right lamp
3. RR left lamp
4. RR right lamp
5. To brake light sw output
6. To Flasher P (pilot/indicator) 3 prong flasher
7. To Flasher L (load)
Third flasher pin "X" to fused hot
Brake light Sw has a fused hot input
Which brings up that you need fuses for almost EVERYTHING on that truck. Old method uses no fuses. New modern method uses fuses and fusible links.
The brake light switch has to run thru the Turn sig switch so one side can be shut off during turn, otherwise it won't blink because blink and stop are on the same filament if you use one bulb.
Original tail light wiring is generally left in place on truck. You will bundle/tie wrap/sleeve down the column thru firewall and route to lamps. Lots of wire along, under and thru chassis.
Be sure all components are 6V compatible.
