I think the bolts on the back that came from Toyota are zinc dipped, not stainless. If you go to the local R&S/Auto Zone/etc. you will find stainless replacement. You also have the option to use weatherproof caps, also sold locally. These caps (about $2 )are two pieces, one half (clear plastic) is put under/on the bolt before you screw it in (looks like the bolt is in a little cup). After you secure the plate with the half cap and bolt, you simply snap the color coordinated cap over the clear cap and bolt head. Also a good thing to do is use a little never seize on the treads of the bolt, so no matter what happens you can loosen the bolt easily.
Koz