night vision has a mind of its own

I purchased a reverse night vision camera off ebay for like 45 dollars I got the one that mounts on top of you licence plate so no modding neaded. Any way When it is working it looks great alott clearer than my last camera. It is only a 2 wire hookup and the night vision comes on automaticly.
My problem is when ever I drive under a bridge or some tress the night vision comes on in the day time. My screen will get either very very bright and cant se nothing but white shadows or it will get verry dark and the only thing I can make out is the clouds. Now at night it will only work some times. I thought my light bulb for my plate was trigering it but I disconected that. Still it gets verry dark and you can only se head lights. Basicly the sucks, the only way I can get it to look good is if I turn the car off and back on reseting the cam.
So I was thinking of hooking a switch to the power or taking it apart and hooking a switch to what ever makes the night vision come on.
Is there a better camera out there that I dont have to rig, or use in an area that has no trees or bridges. Advice on brands or advice on my switch mod is appreciated.

Not able to give much help because I have no idea what kind of vehicle you are working with, but here goes. Also no idea which frame you are working with. If possible flip the camera over and mount it on the bottom of the frame. chances are the bracket that your license plat light is mounted to is shading your camera. If the cam is at bottom of plate this shading will not take place.

I bought 2 off ebay for $94.  I install them in two different cars and both of them give problems.  when they do work  their ok