One of the wagons I had I cant remember if it was the 79 or a 74 one I had a few (Pintos I mean) had a small white plunger switch mounted on the floor about halfway between where the lock is and the drivers side edge of the rear opening for the door. So when it closed it pushed in the plunger I thought that was for the warning light but maybe it was for the cargo light.
If you can get to the contacts I would start with a voltmeter and see if theres any voltage across the two wires. If theres no voltage acros them then use an ohm meter and see if theres conductivity from one to the other or not. Steady conductivity could indicate its a solenoid winding. Some small resistance. A switch would show conductivity when its closed with practically no resistance, assuming the contacts are clean so be careful what you o to us there

Sometimes you can pierce a wire with a sharp sewing needle to get electrical contact to read the voltage or resistance.
If someone has a latch with the wires and can examine it they should be able to get a good idea if its some switch arrangement or if theres a little solenoid or whatever in it.
I thought at times of trying to add a solernoid device to the rear door on my wagon to open it from inside or with a remote of some type.