Detecting presence of a voltage without drawing too much current
Since it appears that the voltage across the optoisolator input is low enough for the controller to interpret it as a low, just put it directly in series with the switch:







simulate this circuit &ndash; Schematic created using CircuitLab

Since the door controller is already limiting the current, you don't need any additional resistors in series with the LED.

If the current sourced by the controller was more than the optoisolator could handle, then a slightly more complicated circuit using a transistor would be required.