Made a delightful discovery the other day on my 80 GS while swapping in a European instrument cluster... a pre-wired harness for power mirrors!
-with the instrument cluster removed, I noted a 6-pin female connector neatly taped to the main harness behind the instruments (pic). I removed the holding tape and found the connector fully integrated into the main harness. It's about 6" long.
The cables are all color-coded various green. Having a JDM-style twin (pic) joy-stick mirror switch I got from a 1980 Leather Sport wreck, The connector plugged right into its harness, color code wires matching perfectly.

I then spotted ANOTHER pair of 4-pin (pics) connectors, one taped to the main harness, with its mate, similarly color coded, taped to the dash sub harness that runs behind the radio and heater controls. They neatly plug into each other, which means the sub harness carries control over to the passenger door mirror. I have not chased down that end of the harness, which will be up by the heater motor I expect. A similar one should be on the driver side. It still means I have to install the actual in-door harnesses, but this is all mostly plug-and-play...
anyway, the serial number on my RX is
SA22C-581600
so if your 80 SA sn is higher (and lower ones may have this feature, too), this harness should be present. There was a harness upgrade during the 80 production run - but I don't know at what ser. no. it starts...
A possible easy-to-check visual hint:
The dash panel under the choke/hatch release button there is a flat cut-out to mount the joy-stick control on later SAs. Earlier ones, the dash is curved with no flat mounting area.

Finding the Joy-stick mirror control might be a challenge, too. For all I know the later 81+ 4-button switches will plug in the same way, but mounting this will be problematic because the dash is different...
Stu Aull
80GS
Alaska