Did you check the relay?
http://www.theschillings.com/boxrelay.htm
Unfortunately, it's the only one of its' kind so you can't swap it with another in your car. If you know someone with the same relay in their Boxster you can swap with them and see if it works before buying a new one.
Btw, if it turns out to be the relay, think about buying the smart top relay instead so you can put it down without the hand brake engaged.