once you gain access to the car, put a set of jumpers on to the charging posts, and open the doors electrically. then remove the jumpers and proceed to replace the battery. be careful when placing the jumpers on and off - you do not want to create a "multiple touches" of the "hot" cables and thus sending surges of electricity through the half dead system. it will kill your radio, and will hurt many other modules...

the process is, connect the ground on the receiver car first, then the ground on the donor car, then (+) on the receiver car and then "hot" onthe donor car - the last step in one single swoop...

if you were charging the car, then keep the RPMs at 2000-3000 for a couple of minutes and then try to start the engine, but since you are replacing the battery, you don't need that - just enough to open the trunk to gain access to the battery...