brake light problem

[scooby3242]

I have a 1973 gmc truck. My driver side brake light does not work. I have changes bulbs and chased the wire. It appears that the passenger side uses a green wire that works and the drivers side uses a yellow wire that has no power when applying the brakes. I pulled the plug apart under the hood just before it goes into the firewall and no power coming out to the yellow wire from inside the cab. What should i check inside the cab? maybe the brake switch that has two wires?