Sorry this may not be that helpful..... but I vaguely recall something like this happening to my 02 way back (which is why i don't remember exactly what I did to fix it). i THINK i just needed to lube my latch. I bought a new one thinking i needed to replace it but when i started looking at it, it was so rusty and unable to actuate smoothly that once the latch was opened when I'd open the door, it never really latched closed properly when I closed it. If you haven't ever lubed your latch, you may have the same problem. This wouldn't explain the specific behavior of it opening when you lock it but lubing the latch is easy to try and doesn't cost you anything.... good luck and don't forget to post back when you get it figured out.