It's the boot button itself that is faulty. Water gets in them and corrodes the switch. I noticed mine doing this last week when the boot was going mental and the button kept clicking. Then whenever i started the car the boot would pop open. Finally it stopped working and the boot wouldn't release. You need to carefully prise the button out of the boot (use a knife), disconnect the wiring plug and then take the switch apart (really easy). Once you've done that you'll see it's corroded. Either replace, or clean it off using some wet and dry paper and some cleaning fluid. Re-assemble and it'll all work fine again.