Where the fuel door release is located on a 1988 190 e?
As far as I can remember the fuel door locks with the central locking and has no extra release. Press the side of the door facing forward when the car is unlocked and it should open.
There's a manual release inside the boot/trunk. Carefully move the inner plastic of the boot (right-hand side) and u will see the fuel door vacuum mechanism, pull the tab to release the fuel door lock.
If the car is equipped with that option there will be a button in glove box. If no button, then just push inwards on the edge of the fuel door and the rear edge will pop out. If glove box button is present and not working, there will be an emergency release inside the trunk on the back side of the fuel door. Usually a cable or a lever.