Outer Limits 4x4 Board

Got a guy wanting me to hook up a shift light to his commodore ute (either VY or VZ not to sure), no after market tacho just the light. Anyone done this and any ideas where to hook up would be good.
I've heard taking a feed from the computer input messes with the engine. Just wanted to know if going to a signal wire off a coil pack will affect it or if it will work? Or should i just try the tacho feed in the rear of the instrument cluster.

The shift light is deactived on Holdens and it is not wired in.
You just have to put an extra wire in the pcm and that is your ground for your light.
Now you have just to actived the shift light in the PCM via hptuner etc.

Thanks mancap, thought there was something like that i could do. Problem is he wants one of those big after market shift lights so its "easy to see" when he's doing his quarter mile runs