Knowing the make and model of your bus would help the discussion with clarity.
However, on a 2006 IC CE300, the easiest way to test the door mechanism is to access the key switch to open the door from the outside. To access the switch and wiring, take the right side of the dash off (6 screws). On my switch, there are four leads, three white and one black. I can open and close the door by shorting one of the side whites (yellow and blue in the picture) with the top white (white in the picture).
You can figure your own out, but that's how I ended up replacing my key switch with pushbuttons inside a Masterlock lockbox typically used for spare keys.