Looking at my technical training books for the Classic....
"Door Position Switch
There is a door position switch, located adjacent to the door hinge,
which identifies the door position at the 13deg position. The switch
position information is for the ETC and will inhibit the APU start if
the door is not >13deg open.
NOTE: Not all A/C will be fitted with the external 13deg position switch"
The switch is labelled as S1598.
The notes do not specify what happens if the door switch is not fitted. However, if I recall correctly, the start procedure specified that the door transit light be extinguished prior to cranking. The diagrams show 15deg and 45 deg switches in the door transit light circuit. The appropriate position switch is used depending on air/ground status.