Rebecca3040

The monowiper on my Merc C class classic is playing up. It sweeps the full length whist in normal wipe mode but stops 3 inches short whist in park or intermittent wipe mode. I have managed to strip it down to motor and transmission but disengaging the motor from the transmission looks tricky , any ideas as to what the fault could be would be of great help. I have a video of it in action but its too large to upload but could be sent in a P.M.