Get in trunk of eldorado cadilac from back seat

You did not indicate if the key turns or not ... but regardless ... it sounds like you have a trunk latch mechanism issue and not a key lock issue ... and your remote cable from the passenger compartment to the trunk is probably broken. Not a good situation.
I am not familiar with the back seat of that car year/model ... but I would try to remove the bottom part of the rear seat to then see if there is a way to remove the top section of the seat. What you are trying to do is gain access to the trunk from the back seat ... since it sounds like your mechanism is jammed.
A low odds of success could be to take some pressure off the mechanism by pushing down on the trunk lid and then trying the key .. probably will not work but it is easy to try

wind the back down with a coat hanger then go in through the back seat,this will teach you to keep a spare key under the car in a secret location wrapped in a oily rag then a plastic bag .Or am i so old to think of such a thing that i remember valve radios in cars made of metal

There should be a manual key hole somewhere in case of a power loss.

Call a locksmith,about 50.00.

Remove the rear seats to get to them.

I have a 2003 camry had the same problem. My solution was to raise the drivers door handle then turned the key and then it opened.

if you insert the key in the lock and turn it alittle further the boot should open.the noise you hear is the actuator moving,but the plastic rod to open the boot is probably brocken.

yes a screwdriver or pliers depending if you can see the lever from where the key fits from .just turn it or open the lock itself

The RSS is located in the trunk on the seat back