Rearview Mirror problems of the 1996 Dodge Ram Van

Moveable side view mirrors slip with shutting doors and with travel at highway speed (>45 mph). I (owner) attempted to repair initial slippage by tightening screws. Mirrors soon began slipping again, and vehicle was past 36,000 mile warranty. Screw threads are now stripped, mirrors won't maintain position. Dealer recommends purchasing two new mirror assemblies (exact same style) for several hundred dollars. Manufacturer rejected request to pay for installation of replacement mirrors from alternate source. Note: I previously owned a 1987 Dodge van which suffered the same problem, although not quite as severely. The problem is clearly due to a long-term failure of Chrysler to address an obviously defective design. Absence of working side view mirrors represents a significant safety hazard, although in this case no accident has yet resulted.