Well there's the white piece on the end of the cable....i'm not sure if there's anything more to it than that..i've never had the cluster out so i'm kinda in the dark.
Try to unbolt the cable from the frame to give some slack, plug it in and rebolt it. Thats what I had to do when i relocated the speedo cable toward the right for my 5 speed swap. I dont know if it would help for the older foxes.
I have this problem every time I plug the speedo cable on a Fox. Only thing I've found that helps it is if you either have someone rotate the other end of the cable until it pokes into there (the square peg into a square hole thing only works if it's lined up), or if you pull the gauge cluster out so you can rotate it a bit and wiggle that bugger into there. I've had to do it both ways, it's a royal PITA for something that would seem to be so simple.