Shift light install on my 96GT - attempt at your own risk
I found another how to on the net that recommended tapping in to the wire labeled "S" on the instrument cluster, and tried this first. And the light did flash at the designated RPM but only if the tach swung by slowly as in grandma driving. At WOT the light never once lit up when this wire was used on my car. I ended up buying a repair manual at a local book store and traced the right wire from the wire diagram of the ECU.
What I did to install the shift light on my car:
The module had 3 wires and 1 plug coming off of it, Red Black & Green.
I connected the Red wire to a + power source that only has power when the key is in the "run" position. I used the same wire I had previously run for my voltmeter & oil pressure gauge but you can probably find what you need in the fuse box. I would stay away from the fuse for the airbags though.
The Black wire was grounded behind the kick panel. I drilled a small hole and used a screw to hold the spade (with the ground wire) to the metal of the car.
The Green wire was tapped in to the orange wire with the white stripe at pin 48 of the ECU wire harness. Remove the door sill by gently pulling up on it. Then remove the kick panel by pulling the retaining pin and then gently pulling straight back on the panel. Remove the computer by removing the 10mm(I think) bolts that hold the harness bracket in place but be careful not to drop the socket or the bolt into the frame rail because it is very hard to get them out if they fall in to that bottomless pit. I did not need to remove the computer on my car but you may need to in order to find the right wire.
Make sure the module is set to the position for the # of cylinders your car has.
Plug the quick light in to the plug on the 5310 module and start the car. The light should flash one time when the car is started. Rev the motor to 3,000 rpm using the 3.0 test pill and the light should come on. If it does, replace the pill with the one for the shift range you want to use and test it again. Make sure the motor is at operating temp before pushing it too hard. If all looks good hide the module under the dash, mount the light and zip tie the wires out of the way and you are good to go.