Here are a couple diagrams that show the routing of the ignition wire that sends signal to the relay to start. Notice the white/pink 18 ga. wire Goes directly to either the neutral safty on an auto or clutch interlock on a manual trans. So it should be white/pink to the clutch interlock then red/lt green to the relay. Now different years used different colors like you say yours is brown/red at the clutch interlock. If your getting 12 volts through out the w/p wire right up to the clutch interlock but with the clutch depressed your not getting any thing after the interlock then the interlock is bad.
I would guess to say that if you jumped the w/p wire around the interlock temporarily to the other side it would crank.