I had same problem with which easily fixed with an hour of messing around, some sealent, spanners, and electrical bullets!
Myself and a mate had to strip out the the tailgate gubbings, pain if you have big hands I warn, you will the find my Ford has some what cut corners on the boot switch sealing and wires! The wires are flimsy and poorly placed, if like mine half cut and rusted! The boot release switch its self has the smallest amount of sealent around it and again if like mine leaked like a sieve and allowed water to pour in and freeze nicely during winter! cut and use ya bullets to repair the wiring and seal nicely in some shrink wrap, take out boot switch and dry out. Apply some sealent around it and replace rubber surround. Replace all and bobs ya uncle aunts ya fanny. Ive had many friends with same issue. All of ours were the 04 to 07 model, mine being an 06.
Since doing this I havent had a singal recurrence of the annoy openning boot while doing 60 to 70 on the motor way.