Automatic antenna on my '97 500sl is not being driven up and down. Antenna motor runs when radio turned on and off. Antenna slides up and down manually very easily.

Mercedes preferred supplier of aerial masts is Hirschmann. There is a connector to the top of the box a coax aerial connection to the mast root, an earth lead strap and a bolted bracket to the base. The aerial mast exits through the car skin in a rubber fitted sleeve. You say that you can hear the motor running but that the mast refuses to emerge. The beauty of Hirschmann is that their motor drive antenna assemblies are readily disassembled (and reassembled). You will need to take the mast assembly out of the vehicle; disconnect the wiring plug socket, remove the bracket bolt, earth strap and unscrew the coax cable connector. Pull the mast firmly down and out of the body work sleeve. The outer aerial mast is driven up by a toothed plastic flexible inner rod normally stored in a coil within the motor assembly. Remove the torx screw in the middle of the rounded end and separate the two halves. At the top is the electric motor set horizontally which drives a rubber belt to a helical drive immediately below it lying parallel. If this belt is broken this would give rise to the motor running but not engaging . The helical drive engages with the edge of the storage drum feeding out and retrieving the coiled rod to raise and lower the antenna. If the helical drive or the drum edge has missing teeth then the aerial cannot function. All these components can be be bought from Hirschmann spares or through your Mercedes dealer.