Where is the coolant temperature sensor located on a 1988 Monte Carlo SS?

It's located on the front of the engine (dead center), mounted in the water outlet. It has a hex head & an electrical connector attached to it. You can use a deep socket or crescent wrench with a short handle for removal.

Here's what it looks like: http://www.autopartswarehouse.com/popup.php?type=m...

Assuming that this is a 305 v8 or 350 v8, it should be in the side of the left(passenger) cylinder head between the 1st and 2nd park plugs. Those are actually the #1 and #3 cylinder's spark plugs.

Hope this helps.