i had a similar query quite some time ago and got the same replies. The only thing that can be done is to change the resonant frequency of the rearview itself. But even that will yield only minimal results. The cardboard idea is pretty well bunk, as all it would be doing is adding a suspension component, or even a less effective gasket component. Depending on the car, there really isnt much you can do to mitigate this especially if the rearview is directly connected to the windshield. sometimes if its connected to the ceiling, then you can do somewhat better if you stiffen the sheetmetal around it or just deaden the fuwk out of it