Towing Mirrors

http://amzn.to/2o4e9x3

Possibly cheaper alternative http://www.ebay.com/itm/361016270378

Basically, the base of the mirror consists of an aluminum casting that is supposed to attach to a GM truck via 3 studs. In a nutshell, you have to grind away some of the casting that interferes with the Astro door, then transfer the 3 bolt locations to the door and drill new mounting holes. Replace the studs with longer bolts. Lastly, depending on how its positioned exactly, you may have to shave some plastic from the leading edge of the cover around the aluminum base..... this may rub on the body when the door opens.

Here are some install pictures from AKforceten: