Waylander is correct (assuming the S2 convertible is rigged the same as a 968 convertible). Removing (carefully) the vent in the B pillar visible when you open the passenger door should reveal a wire cable with a loop on the end that releases the boot. Failing that, get underneath the car and find the big wire on the starter motor, connect the pos of your jump leads to that and the neg to a suitable chassis place, that should get power to the car to release the boot via the button in the drivers side footwell - although you may have to turn the ignition on to energise that button.