I did exactly that on my Mustang. Trying to make it as much of a sleeper as possable, I iwanted to mount a shift light somewhere unnoticeable, and without a 5 inch attention grabber nailed to my dash. This is what I did... MSD sell an RPM activated switch(P/N 8950) that will turn anything on or off at a certain RPM. I stuck that in my glove box so I could have easy access to it so I can change modules when need be. After than I took aprt my instument panel to find an unused idiot light slot. Since my car is Carburated, the check engine light was not being used fo anything, so I plugged a light into that hole, and wired the whole shebang together, and Badda bing my check engine light glolws whenever it´s time to shift.