I'd get another cluster and see if you can get a working speedo out of the two you'll have. These instrument panels aren't expensive, used.
You've pulled out the little silver drive motor and verified that the tiny tan gear under it is intact? THis sounds like a classic case of gear failure. It can be a little tricky getting that speedo out and apart...
On my 91, I believe the one extra wire was for the SRS lamp. I thought another lead went to a double terminal in the center of the cluster behind the speedo- and it looked like a ground lug on the cluster, but it may not be. I thought it was a yellow or maybe blue and yellow striped wire. It's been a little while.
As Chuck noted, there's a white/red striped wire there, for the tach. Don't plug it into anything except the tach if you have one. The end is insulated so that it can be left disconnected.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K