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.
go to www.summitracing.com They have it for like $50-$60. They are usually the cheapest place to find good parts like that. Just type MSD-8950 in the parts number search. You will also have to buy RPM modules. Thy usaully come in 5 packs in odd or even rpm increments. (5000, 5200, 5400, 5600, 5800, or 5100, 5300, 5500, 5700, 5900)