I had same problem on my 166 but not sure if the set up is same on 164. Since the motor is running then it is likely to just be the actual wing mirror is loose - tighten the screw / allen head underneath the mirror as this makes sure it is sitting down on a small lug which actually creates the movement. It is easy to take off wing mirror - just remove the plastic cover on inside of car (one allen head screw), then there are 2 / 3 allen bolts inside frame. Once everything is put back tight, it will be fine. I think car washes knock the stuffing out of them and also if you use it regularly then it comes a bit loose.
Hope this is of some help but I really dont know the set up on 164.