tfbmiata

HI All - wasn't sure if this is the right place fo this but I picked up a used shift light and can't get it to work. It powers on, goes through the initialization sequence. It only has 3 connections, ground, power, and tach/ig. It says its compatible with 5v and 12v tach signals. I've hooked it up to the obd diagnostic ports (referencing the setup guide from flyinmiata for the right pins) but can't seem to get it working. I tried the cables in the back of the gauge cluster with the same result. I broke out my trusty multimeter and found that the v reading for tach signal in the diagnostic power is like 10.2/10.3 but doesn't fluctuate with additional RPM. I retested this on the gauge cluster wiring and it reads 12v, but again, doesn't really move at all based on RPM. I was under the impression it is supposed to move based on RPM and I'm guessing this is why the device isn't working. Thoughts/opinions/helps?