If you hook a battery to the car you should be able to actuate the locks by moving the door lock button up & down on the drivers door. If that doesn't work, the easiest way I have found is to remove the rear seat cushion (just push the red levers at the bottom of the seat toward the outside, lift up, then pull towards you). Just inside the right rear door you'll see a yellow vacuum line under the insulation. If you carfully cut that line & apply pressure gently ( not too much or you'll destroy the diaphrams) to the half going into the trunk, you'll unlock the trunk & fuel door. You'll need to use some sort of pump or compressor, just blowing into the line won't give you enough pressure. You can then just splice the line with a piece of rubber hose.
Unlike the 123 bodies, the 126 uses a 12 volt vacuum pump/compressor in the trunk to actuate the locks (vacuum locks, pressure unlocks). The engine does not have to be running.

__________________
82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K