IDK if this really matters sence its just a run-of-the-mill aftermarket tack, but here are the vehicals I'm working on
1988 Chevy C2500 350/700r4
1991 Chevy K1500 350/700r4

I installed a Sunpro tach in my '88 a while back. I just got a '91 4x4 and everything on the '88 is getting put on the '91. when I installed the tach apprently I did it wrong. the RPMs were acurate and all, but the back light has never worked. I checked the bulb by putting it in my blinker and it works. I had the illumination wire hooked to a white wire on the brightness wheel that, according my volt ohm meter, only got power when the lights were turned on. So, what wire is it supposed to be hooked to?