Hi Donna, My 2002 mondeo used to do that after heavy rain, I never found a solution for it as it just stopped, I could only open the boot with the remote fob after it stopped opening by itself though.. I was looking on a mondeo form and they say it is the boot lid switch above the rear licence plate.
Copy and paste this http://www.fordmondeo.org/forum/showtopic.php?fid/243/tid/791757/ so you can see for yourself what they say or even register and ask them for more advice if no one else here has the answer for you..
Good luck
R
R, March 2010