As in a lot of instances the manuals don't really tell you the way things are in the real world.
Firstly, am I right in thinking this is the car you've just fitted the s/h gearbox to? And has the fault happened since? If so I would check the cable routing.
As for removing the head, if memory serves me correctly, you remove the warning light cover, behind which are two screws which you remove then gently free the speedo a little towards the steering wheel. (Not too far.) Then reach up into the glove box and the end of the cable has a sleeve approx 6" long which you undo which releases the cable from the angle drive on the rear of the speedohead. Refitting can be done by pulling the cable through until you can get your little pinkies behind the speedo to refit the cable/angle drive.( Or the pinkies of the child labour you're using)
Test the angle drive while it's out, and spin the end of the cable in the speedo to see if that's working. All of this can be done without disconnecting any wiring.
One futher word of advice would be to disconnect the battery, if you wear rings, or work with spanners/tools behind there, it could cause a short.