How do you open the trunk of a 1998 xj8 jaguar when the battery is not dead and the key wont open it?

You remove the licence plate. You poke a hooked coathanger wire into the bottom right slot and hook the a steel wire that runs above the slot. Pull hard enough to bend it. There's two in there- it might be necessary to try them both. After you get it open unscrew the carpet covering and unbend the wires and fix the key turning parts, etc. before closing the "boot" again.

This is the best way into an XJ8 stuck boot. Use a slim screwdriver through the right hand bottom licence plate hole. Hold the face of the screwdriver flat at a right angle and lever it gently to the right when you feel resistance. You will feel the internal rod springing back and hear the motor. If the boot still resists, press down on it as you lever the spot. The likely cause will be the wiring loom at the right hand boot hinge. You can lever off the plastic cover to reveal the loom but it will hard to spot any breakage. An auto electrician charged me £35 to fix mine.