The 240 doors are easy to take apart. I assume you've got speakers behind the door panels the way they're supposed to be.
Remove the cover from the window crank and the crank itself (phillips head) if it's got manual windows.
Remove the lock knob. Remove the rectangular trim around the door release.
Pry out the two round plugs in the door handle. Two big screws under there- phillips on early cars, large torx after 91 or so.
Pull the door handle out a little bit, remove the power window switches if it's got them, and rotate the door handle clockwise so it's straight up (CCW on passenger doors). Pull the handle in towards the car to unplug the pin at the top.
Remove the door pocket- 3 quarter turn plastic fasteners, then pull up in the front, up in the back, and pull the bottom away from the door panel to unhook it. Remove the big screw and knob at front and rear of door.
Now with a big screwdriver go around the outside edge of the door and pop each plastic pin out as you come to it. Soon the panel is just hanging by the top edge. Pull it up and twist a little to get it free of the mirror handle. Should come right off.
Now you can get at the speaker wires and speaker and see what's going on there- it may just be a bare wire grounding against the door. Replace the speaker if you need to and just watch for clearance to the window mechanism.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 220K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K, 88 244GL 145K