Everyone knows in order to mount a vacuum base holder one need a perfectly smooth surface. And since there are not so many such surfaces in your car most people usually mount GPS/cellphone holders on the wind shield even though it is not always the best place for it.
Actually, we can attach a vacuum holder to quite a rough surface with the help of reusable adhesive material called Patafix.
In case you are not familiar with it it's very useful stuff with a texture of an (already chewed) gum. It easily sticks and un-sticks just like Post-It notes and it also can be bought in Office Depot.
Apply 1-2mm (0.1") thick tube of the Patafix on the perimeter of the vacuum cup.
After you press down the holder the material will fill the small bumps on the surface and hermetically seal the cup.
Note that the Patafix was applied not on the extreme outer edge of the cup but rather a little bit closer to the center. This is because there is a plastic cup above the silicone vacuum cup and we need to have the Patafix under (on this upside-down picture it's above) the rim of the plastic cup.
Thus when you press the plastic cup it will apply greater force to the layer of the Patafix.
Attach the holder the press on the plastic cup as hard as you can to let the Patafix fill all the bumps and pores of the surface.
Hold it for a couple of seconds then without letting it go apply the lock.
The holder is now mounted as firmly as if it was attached to glass. Notice that even the notch running below the holder was perfectly sealed.
Patafix is an excellent material which can be easily removed without leaving any traces even after being exposed to extreme heat of the car parked under direct sun light in summer.
This hack allowed me to mount my cellphone holder so close to the driving wheel so now I can dial without even taking my hands off the wheel and my eyes off the road. But, please, don't text and drive!