Removing instrument cluster

The release finger on the cable end is best described as one half of a wooden clothes pin. If all of the plastic broke bad enough, the cable will put straight off and need to be replaced.
If not all of it broke and the very end that catches the instrument hasn't broken, here's a plan. ... Jack up the car enough to go under the drivers seat toward the trans. You'll find a sheet metal strap welded to the floor that captures the speedo cable. Free the cable out from under that strap. Now you should be able to pull enough cable length inside the cabin to unhook the cable from the instrument with a small screw driver.
I always do this anyway. Speedo cable is about the last thing I'll disconnect and the first thing reconnected. It serves as my helping hand.