Electrical Question

If the tach has stopped working, it's most likely done for, and there's not anyway I know of to fix it. Might be time to start watching Ebay for a nice used one.
It's easy to remove the cluster, but all the connections are made through a single plug. Just drop the lower dash and the shroud around the steering column. Then unscrew the hood that surrounds the cluster. Of course you'll have to pull out and unplug the switches to either side of the hood. Once that's off it'll be pretty obvious what to do. Oh, you do have to unclip the speedo from behind the dash though. No worries, it's not hard, just tilt it to one side and it will slide off. Just push it on straight to put it back on.