[joedoh]

as an experiment, try adding some mass. vibration is energy and there is a point where the vibration is the highest with the smallest input, it is called the resonant frequency. if you can lower the resonant frequency value, it will take more energy to vibrate it.

this is the theory that sound deadening material works on, adding the heavy mat to the panel lowers the resonant frequency and requires more energy to vibrate. lower frequencies take a lot more power and arent as intrusive as higher frequencies, think of the last subwoofer amp you saw, probably twice as big and three times as powerful as the mid/high amplifier. amazing how its all correlated to the same theory.

anyway, as a temp thing to try out, try attaching some weight to the mirror head and see if that helps. stop at the sporting goods store and they sell little stick on lead pellets with tape on the back for adding mass to golf clubs. if it works, add some weight permanently. maybe open up the mirror and put the lead inside. maybe pour lead in the arm tube? any way you can to add mass. add some mass to the base and the inside of the door where it attaches too, like asphalt sound deadening material.

it may be impossible. I put a long lokar shifter in a truck and it looked great sitting still but wiggled like a widow pleaser when the engine was running, just how long it was and its solid attachment to the transmission made it really susceptible to vibrating.