I have a 98 disco 1 , the rear cargo door seems stuck the plunger moves up and down with the rest of the doors when unlocking.... how the heck do I get it open... tried inside handle and outside handle

 

Take the oval logo off the exterior handle. You can pry it off with a small screwdriver. Then spray the linkage inside with wd-40 or equivalent.

 

I have had this happen 3 or 4 times. Use lots of WD-40 and wait about 20-30 min, it will open like it never had a problem. Always seems to happen when the back end is full of stuff too. Don't forget to wipe down the door afterwards.

 

Even though you can see the plunger move up, have you tried using the key? I had to stick the key in the cargo door tumbler and turn as if to unlock it and hold the key in that position. I could then open the door.
If so, you need to replace a wee little spring in the latch assembly.

 

Thats what wrong with mine. It looks like a royal PITA so I haven't done it.

 

It wasn't too awful bad. I busted most of the push retainers getting the panel off. Took plenty of pictures. Was patient with the little clips (didn't break a single one ).
I suggest getting a handful of the push retainers when you get the spring.