Went back to TRIPLE check the connections and low and behold the problem is on the tach itself. The ground post is janky. I thought something was weird when I went to put the wire connector on that post and there was no locking washer like there was on the other two. The post spun when I tightened down the nut to hold on the wire connector, and now that's it's actually in the car I can wiggle the post and spin it. The bottom nut is tight but the post is still loose. I spun the whole post setup like a half turn(with my fingers and not a wrench), started up the car and the tach is now working. I think something or someone over torque that post from the factory and whatever it is touching inside is now compromised. Don't really want to take it back out to return because its a pain in the ass. Maybe I'll get lucky and my half turn will stay put.