Cant get trunk open on 1998 cadillac sts ,i tried the botton and key.neither which work,

The fuse is located under the rear seat in the fuse box.Trunk/valet fuse.

It actually sounds like the micro switch in the drivers door that reads the key fob is bad, if the key works and the fob works on the other doors and the trunk. You will most likely need to have the door handle replaced, they're not very expensive by themselves but it can be a lot of work. Cadillac CTS Door Handle Repair Micro Switches Replacement 2nd Gen The video is a 2008 cts but will give you a good idea of whats wrong and whats required to fix it

look at the latch and see if it is hit in the right spot

I would check the owners manual or with Cadillac - its possible the remotes lost part of the signal and need to be reset. It sounds like the receiver is not getting a signal from them by using that button.
Could be the receiver too.

There is obviously a problem with the trunk release solenoid in the trunk. You will have to do it the "old school" way and use the KEY to get the trunk open. Once you get it open, inspect the wiring conduit that runs between the body and the trunk lid. You will most likely find the problem there where the conduit bends back and forth each time the trunk is opened and closed.

Hello: On the rear bumper, driver side lower portion you will see a square which you can flip open with the key you have. Once open you will see a lock cylinder, insert the key and turn. That will open the trunk. Inside the trunk you will see a handle, pull it and it will open the drivers door. That should do it.

Does the key you have open the driver door? Do you have a keyless entry key fob, does it work? you also might try removing the glove box and disconnecting the wires from the trunk button and touching them together with a paper clip or other small wire but before you do that look in your owners manual a determine which fuse is for the trunk release and check the fuse then the last would be to have GM dealer check to see if the key you have is the proper key fror your car and if it may be worn out

I am in the same situation! Did you ever get the part number?

o.k. in the glove box there is a valet switch that usually gets hit by something, open glov box and flip the switch then try button again

on my 1998 Deville its located in the trunk. the panel thats is butted up to the back of the rear seat. look for some black nuts around the top of rim of the trunk, they are about 1'' long, these have to be removed first... good luck.....

Mac.........