Stupid question . . . is it unlocked? There should be a knob inside like the other doors. If it is not opening from the inside, the latch mechanism may be broken. There is a little pot-metal casting that the inside lever screws to that is known for breaking. But if this is broken, the lever feels like it is just floating in the recess it is in.

Even if you can not unlock it with the key from the outside, it should still open with the button on the outside if it is unlocked. Also keep in mind that it will only unlock with the master key which has the square head not the valet key which has the round head.

Mine would not unlock with the key, so I pulled it apart and found 21 years worth of road grime that had blown into the cylinder due to it being on the rear of the car. I cleaned it up, lubed it with marine grease and put it back together and it works great. Be warned that if you have to take it apart, it is difficult and you will need universal joint sockets because it is not a straight shot to the nuts.

__________________
Doug

1987 300TD x 3
2005 E320CDI