How do I open the boot if the battery is disconnected ?

SOURCE: recharge battery when boot is dead locked

Hi, see if this site helps, it explains how to access the battery (who designs these things???). Let me know how it goes.
http://www.ppbb.com/boards/ppbbphp/showflat.php?Board=ARCHIVES&Number=1225855

SOURCE: 1999 porsche 996 replaced key fob battery

Assuming the car battery is ok (if not, buy a male to male cigarette lighter at the auto store and connect to another car until charged enough to unlock the doors...but don't try to crank the ignition!). The trunks won't open if the battery went dead with the alarm set because arming the alarm physically locks the releases.

Sounds like the key code is switched off because you replaced the key fob battery. Try unlocking the car manual with the key in the door lock. Then switch the ignition on to reactive the keyless entry. If this does not work, you might also try pressing the fob unlock button with the key in the door lock until the dash led flashes.