I have NOT experienced this other than when the battery was completely dead - at which time, as page 39 of my '99 Owner's Manual states "To unlock and open the trunk lid, turn mechanical key to position 1 (about 10 o'clock), hold and push to open."

Assuming that the Remote, and console mounted Release Switch, and the Trunk Push Button have all worked recently, and the battery is fine...
It sounds like the ?motor? that locks the trunk has died and it remains locked, even when the doors are unlocked.

If it's not simply a wet and frozen mechanism, (that a good sunny day may resolve), my guess is that you'll need to access the trunk by removing the rear seat back(s).

The method of removing your seat backs probably varies depending on whether your seat backs are fixed, or fold down by pulling levers in the trunk. (My '99 C43 has fold down seats, my '99 C280 Sport does not.)

On other cars, I have removed seat backs by first removing the seat cushion, and then removing the exposed bolts holding the bottom of the seat back, then carefully lifting the seat back to clear.

Page 141 of my '99 Owner's Manuals state to remove the Rear Seat Cushion:
"Push in locking tabs (on left and right side of seat) and pull up seat at the front.
Caution!
Watch out for sharp edges when removing or installing the rear seat cushion, and make sure that it is securely locked in place again, to prevent personal injury."

Once into the trunk through the seat area, it looks like the simplest solution will be to remove the stationary latch that the lid latches onto. Below the lid mounted latch, remove the two vertical elongated plastic covers, and then remove the two bolts fastening the stationary latch, and I believe your lid can then be raised.

(Disclaimer: I have not done this, but only quickly looked at mine. If no one more experienced with W202s can advise, I can further check either of mine after you tell me which seat back type you have (stationary or fold-down).)