Hello i dont know if this helps but i had a similar problem and it was my grounds , check for the ground wires behind the gauges and generally any loose grounds and any shortages.

 

So I was able to get the cluster out and get a new voltage reg installed. However, when I put the cluster back in the tac doesn't register. Ideas?

 

If your tach was working before pulling the cluster then is is something that you messed with. Pull the cluster back out and check all the connections and the backside of the cluster. Something was pulled loose when you were back there.