Gary Casey wrote:

...The simple way might be to do as suggested and use the third door
contact to complete the circuit.  The light would go out if the door
was EITHER latched or fully open...

Gary,
As you suggest, I found this to be both simple and pretty foolproof. The
"door unlocked" light will go out with the door open about 6 " at the
bottom--depends on how you adjust the electrical contacts--and I thought
that also served as a good reminder. When closing the door, check for the
light--it indicates that the circuit is active and functioning. If the light
goes out when you latch the door, all is well. If it doesn't, you have a
problem.

Using a pin set into the "head" of the hinge operating shaft and using this
to activate a microswitch that's mounted on a small scrap of 1/16" angle
aluminum was much easier than I first thought, but the most important reason
for mounting the sensor switches this way was the ability to monitor the
"over center" rotation of the latch. In my mind, this was the critical
function that needed monitoring. This is especially important on the IV-P
where you are trying to ensure that 8 latches all engaged fully with the
operation of two locking handles.

Bob Pastusek