Ok...I gave some bad info in my original posts...somehow I got confused from all the stuff I googled.

It actually has 2 red wires, one yellow, one blue, one black.

I found someone on another board from 2006 with a tach that looks the same...but I pulled the back off and mine looks different inside. Mine had a brown circuit board inside and it had a little dated slip of paper inside that had the year 1968 on it. (seemed early for a circuit board doesn't it?) but I could only take it apart so much without breaking it. I'll post a pic tonight.

Here's the link to the forum talking about the same thing...but how do I know which red goes to ground?. And I can't see if one red goes to the case or not like that pic shows as my tach is different inside:

http://www.hotrodders.com/forum/tach-wiring-help-91971.html

It has a dial switch inside that lets you choose 6 cyl or 8 cyl.