You have to first make certain that you've pulled the cable as tight as you can where it comes thru the firewall, you need every millimeter of slack you can get, then grasp both sides of the cluster and pull it straight back gaining every millimeter you can. You pull straight back until just before you think the cluster will break!
The cluster is tilted either left or right; I usually tilt the left side forward and disconnect the multi-pin circular plug with a small thin screwdriver used to carefully pry the connector off the pins. Then you can more easily (a relative term) reach your hand behind it to unscrew the speedo cable collar! Yeah it is very difficult to get your hand in there and yes the amount of force you might need to break that collar free will challenge your thumb and forefinger strength! The one caution is with the cluster tilted either left or right be careful that you don’t gouge or create a dent or crease in the dash surrounding the cluster area. The side and back edges of the cluster have some sharp edges and if you lever the cluster over too hard you can screw up the dash’s vinyl covering. I usually drape a terrycloth towel or something like a sweatshirt around the cluster’s back side and edges to avoid that problem.

That is why some people try to remove the left hand vent and then the hose behind it is pushed out of they way so you can reach over to the cluster, I've never been able to get it done that way, some people say they've been able to reach from below I’ve never been able to do that either!

Wait until you have to put things back together and reattach the speedo cable screwing the collar into place then getting your hand out!