Did you try this fix:

"This is a very common problem and sometimes a nightmare to diagnose but because one mirror being faulty generally effects the other. This usually leads to replacing the incorrect mirror. The problem lies within the micoswitch inside the mirror.

If you're feeling confident and fairly brave you could attempt this quick fix but I accept no responsibility for anything you break.......

Remove the painted mirror back very carefully (you need something thin and strong to get behind it but be careful as these are easily damaged). This will reveal a blue wire and a white wire which are connected to a small black plastic box which contains the microswitch for the folding mirrors. Remove this cover and there's a U shaped piece of metal inside, straighten this a tiny bit and operate the mirrors. Keep trying this (without breaking it) until they fold ok everytime."