He's probably talking about the light green/red tracer wire that runs from the instrument cluster (charge indicator light) to the alternator. You should be able to intercept that anywhere (in the cab or in the engine compartment). I'm not too sure that connecting there wouldn't mess up the charge indicator light. If it doesn't mess whith the charge light, what you'll have is an "alternator hour meter". If the alternator craps out, your hourmeter won't register.
If it was me, I'd just connect to a hot-in-run circuit and be done with it. There should be a light blue/pink tracer customer access wire at the lower right of the fuse access panel under the steering wheel that would fill the bill.
