Here's a nice diagram someone recreated that I saved from somewhere on this site a while ago. For a '78 it's basically the power comes straight to the STOP fuse to the stop switch. That should be an "always hot" feed so if your tester doesn't light up when touching the GW wire then check both sides of the fuse and make sure it's hot on both. But also note the GW wire where it tees off and goes to the hazard switch. If the GW is hot at the hazard but not the brake switch, then there could be a melted or broken wire where I put the yellow dot?
