If you did a search on "door panel removal" you would have found this thread. vinamg gets the credit.
"For 124 you have to do the following:
1. remove handle for mirror adjustment (driver side only) it has a hidden lock that slides into the handle
2. Remove triangle piece, by sticking something (flat and hard if metal you can cover with something) at the top portion and pushing out
3. remove seat adjustment knobs all three by simply pulling them out
4. Behind knob for back adjustment you'll see a small plastic ring remove it
5. Now remove the black cover by pulling top left corner a little bit to clear knob mounts and then pulling upwards
6. remove other black cover that goes around door handle by just pulling.
7. under the cover you'll see a small foam piece pull it out a little and you'll see a 10mm bolt remove it
8. remove seat adjustment box by removing two screws
9. remove door handle by pushing forward and out disengage rod
10. remove the plastic u that Blackbenzz mentioned on the door latch small screw and then pull out bottom a little and then downwards
11. this is hard to explain, inside the pocket you'll see a hidden lock that slides (bottom of pocket) slid it into release position
12. if coupe remove chrome piece in top left corner.
13. Now you are ready to remove panel just remember there is are no snap-ons like the newer models your is always up and out.
There are numerous hooks in the back that require you to push panel up and out to disengage. When putting back pay attention to top of panel that it engages properly into the window channel. Lower the window for both removal and installation.
Now, for my own addition. There is one step that I don't think is explained clearly and it is the part I struggled with. After unhooking the latch rod from the handle (the chrome handle that you pull to unlatch the door), you have to push the chrome handle and it's black plastic housing forward and then out to remove it. For me, that was the key to get the panel off. Then it really is just up and out!
Experience is something you don't get until just after you need it.