Be sure you are using the black key and not the green valet key.

 

In light of the valet key, when I first got my XJS, had only one key. Had a duplicate made at the local dealer, and it would not open the trunk. And both were black. So, maybe a series of re-cut keys. Anyway, got lucky, contacted the past owner through paperwork in the car, and he sent me the whole set of keys, including the original remote. Anyway, as a last resort, a dealer can cut you a new key by using you VIN #. Keith.

 

Should be able to crawl through the passthru of the rear seat and once inside the truck, use a screw driver or pair of pliers to pop the truck that way. I have done this on a few other vehicles where the truck would not open.

 

Just open up the hood, connect jump box to power stud by fuse box and other to ground. Then open the trunk and replace battery as normal.