Thought: try to adapt the manual adjustment mechanism from a Mercedes-Benz W124, W126, or W201 chassis? The driver's side were manual (up to about 1992 or so on W124 and W201) with an adjustment arm that protruded into the little "sail" panel on the door where the mirror adjustment switch is on the 986. Use an RHD part to remove the switch (and LHD for RHD cars) with a grommet to finish the cut hole.
Might not work but if it could be adapted that may save a lot of time and money.
Plus the mechanisms would be available inexpensively in junkyards for prototyping.