How do I unlock trunk release?

If you have the full set of keys that came with your car you should have two that look similar but are different! One is the drivers key and the other is the valet key. The idea here is you lock your stuff in the trunk and lock the truck release near the drivers seat (on the floor) so your stuff is safe while you are enjoying your self. The valet key will only open the doors and steering wheel. Of course if your car is driven off that doesn't help you ;-}

Living in the city I would track in salt on my shoes in the winter and I did manage to mess up my lock!

I had to spray it with WD40 a few times to get the cover to open and even needed to use distilled water to wash out the salt using a can of can'ed air to blow it out and then use WD40 again.