Ah crap, you were right all along, it has a pickup in the trans for the speedo. What MB says to do is remove the pickup from the transmission and then take the broad side of a large screwdriver (honest!) and tap the end of the pickup rapidly and see if the speedo needle jumps or not (need a helper to watch obviously). This is simulating to the pickup what the trans is supposed to be doing. If the needle jumps, the pickup is fine, the problem is in the transmission. If it doesn't jump, then pull the cluster out and find the black 4 pin connector on the back of the speedo. With a VOM set to AC volts, 0 to 3 volt AC range, connect to pins 1 and 2 on the 4 pin connector, and repeat the "tapping" part of the test. If the voltage "jumps" when you tap, then the problem is the speedo itself. If it doesn't jump, then it's the pickup at fault. The pickup isn't available seperately (It wasn't when my book was written anyways), you need to replace the transmission electrical harness then.
Gilly
ps Sorry about the confusion, I really was sure this had a speedo cable. Learn something new everyday, if I remember half of it I'm doing OK.