Hi ..

You need to check for the Electric mirror subloom. This connects up at each lower A-panel (conical connector) the RHS mirror (motors and heater) the LHS one, and the mirror control panel, clipped across the bulkhead with the other main wiring - The other sublooms (if fitted) are Heated screen, and Central Locking control. If the wiring isnt fitted, you can add quite easily. You have a heater wire (Black/yellow) Power (red) from fuse3 maybe, and an earth (brown) to connect up.
All this is pretty straightforward. The hardest thing, if you have to fit a subloom, is dragging it across through the clips, especially behind the steering column reinforcement. I think you'll find you dont have the wiring subloom fitted.
I fitted all of the above lot on my '86 van, along with a Turbo dashboard, and it's cool to get it all working. Tip .. Keep the A-panel connectors silicone-greased up ; )))

Good luck!