I wouldn't carry my cellphone or a camera back there. There is too much moisture in there. You will also end up smashing them up with all of the bumps that you will hit on the trail. I would get a bag that goes behind the windshield to carry valuables like that.
I carry both phone and camera but I keep them in a case then put them in the trunk. I never had a problem. I did this summer buy a windshield bag and will keep them in there. I need the extra space in my trunk.
Well hate to be blunt but, I guess it depends on how much you want them to keep working . I know i sure wouldn't put anthing electronic back there. I carry my camera with me around my neck on a neoprean neck strap. They cost about 15 bucks. The neoprean takes up the shock of the camara and keeping it inside your coat will keep it warm. I have a EOS Digital Rebel and have no problam with it.
Your trunk is popping open?? I never had that problem, but for sure I would not put anything valuable in there. I keep my cell phone and camera in my pockets. Your stuff is bound to get banged around and wrecked, so be careful. The only thing I keep back there is my trail maps.