Sorry but I'm pretty dumb on what actuates the shifter (I assume it's a solenoid?) but I do know electronics very well so I'll try to help. Do you have any idea how much current it takes to actuate the shifter? If you have a current meter you can test it. Most likely the current supplied to power the shift light in the tach is very low so if it takes very much current (probably over 100 milliamps) to actuate the shifter the tach won't do the job. But you could wire in a small relay like one of the type they use to control an electric fan and have the shift light trigger the small relay and it triggers the shifter. I could draw you up a schematic and scan it and post it if you like. It wouldn't be hard at all to wire it. The only thing I can't guarantee is the accuracy. Most likely you'd end up setting the shift light a couple hundred rpm's lower (just an approximation) to make up for the delay in actuation since 2 items have to be powered up.