If you will PM me I will send you a line drawing of the circuits for a 7F, non TWC truck, and a couple photos I took of a 7F truck in our shop that happened to capture the rev switch in view.
The rev switch is an orange/brown colored little plastic box found only on non TWC 7F models. The box is situated behind the left step insert panel. It may be the same or similar set up on 6F trucks.
The later 7F trucks with TWC and ECM do not have a rev switch, the ECM gets the tach signal on those.

But you need to make sure the tach signal wire is intact and sound before jumping to replace a rev switch. To my knowledge, we have never encountered a failed rev switch. (But I bet we will now!)