I assume this could apply to broken trafficators (European Turn Signals) as well:
On my model outside rearview mirrors I replace the support stalks with small diameter Evergreen rod. The Evergreen is far more flexible and forgiving than the much more brittle model plastic at this small size.
I drill a small hole in the truck body and glue in the Evergreen rod for better support. The plastic formula of the EG plastic is soft, flexible and very forgiving. I myself have bumped into these mirrors and they usually just spring back into shape with no damage.