How does a garage door motor wall control on a two wire circuit determine between door control and light control?
I haven't played with a Genie unit, but I spent some time with an oscilloscope and a Chamberlain/Lift Master unit several years ago. I found it has two modes of operation. One is the simple contact closure -- just short the two wires together and the motor will operate. The other mode is digital. The two wires carry power to the wall control, but they also carry a serial data communication protocol between the motor and the control. The only way to control the advanced features, such as turning the light on and off, is to use one of the wall controls with the digital interface.
As I recall, I also found that the two modes can't be used together. For example, you can't put the fancy wall control in one place in the garage and put a simple doorbell type switch in another place. When the simple switch shorts the wires the door operator goes into &quot;dumb switch&quot; mode and the fancy wall control ceases working (until the door operator is power cycled).
I couldn't say for sure whether your Genie works the same as that Chamberlain did, but I would not be surprised if it does. In any case, you wouldn't get far with discovering this nor reverse-engineering it armed with only a volt meter, unfortunately.