Please help. I need a rpm signal for shift light.

I'm having a problem wiring up my shift light on my 90 GT. It was not wired up when I bought the car recently. The previous owner told me when I wire it up to put red to switch power, black to ground, and green to coil. But everywhere I read I've seen where people say if you have a MSD box (which I do...either the 6 or 6AL) you must tap into the rpm signal from the msd box. with the shift light's green wire. He even told me that people would try to say to wire it to the msd box but to not listen and instead, to wire it to the coil. Please help. I tried wiring it up to the negative on the coil and light doesn't work. It does come on briefly when you turn the switch on so I know the bulb and all is fine. So what is the official answer? Is there ever any exceptions to the "gree wire to msd box(if you have one)" rule? Where is the rpm signal on a MSD 6? Thanks for your time and help.

Thanks. But exactly where is the port and how do I tap into into it?

i just looked and its a MSD 6AL model 6420. Does it have a port?

alright...i got it! there was a port on the opposite ends from all the wires coming out of the box. I just crimped on a female end and shift light is working perfect. Thanks alot to the people that helped me.