[Android Head-Units] JY-UL135N2:
OK, I just did a bit of googling and found a thread on the joying forum about steering wheel controls (SWC). This seems to confirm that the Key 1 & 2 inputs are resistance based. The wiring seems to go (something) like this...

Each switch has a resistor in series with it. Each switch/resistor pair are then wired in parallel with the others. One side goes to ground and the other to Key 1. Key 2 can be used for an additional group of keys wired the same way, only wired to Key 2 instead. There may be an additional resistor on the parallel leg of the circuit too?

Sorry i can't post a link to the joying forum thread as i'm a new member who is blocked from posting links. A couple of relevent quotes from the thread...

"So correct me if im wrong. Steering wheel key 1 is just measuring resistance to ground to assign a button function to it correct? Same with key 2 i would guess. And what are the acceptable resistance ranges and how much of a difference in resistance do you need for it to work? I am making a remote control for the passenger side of my rv and was hoping to confirm specs prior to attaching it."

"I was right made up a circuit 75 ohm then 10 ohm increments seems to work fine."

"This particular Outback has two wires coming from the OEM steering wheel controls (SWC, 6 buttons) via a 24 pin plug. Wires are labelled SWC+ and SWC-. They provide a resistance of 4.7kOhm with no buttons pressed or other, specific resistances for each of the 6 buttons on the steering wheel. There is no ground reference.

Initially I made the mistake of connecting SWC+ to Key1 and SWC- to Key2 thinking the unit would measure across both. In this configuration the unit did not recognize pressed keys though. The reason is that the unit is trying to measure resistance to ground and not from Key1 to Key2. Key2 appears to be just another input incase there are additional controls from the wheel. The solution for my car is to wire the SWC- coming from the steering wheel to a ground instead, say battery ground. The SWC+ remains connected to Key1 and this way everything works fine with all 6 buttons recognised. "

I'll need to pull my head unit and extend the Key 1 & 2 wires and do a little of experimenting to figure out the resistance range and steps needed for the head unit to recognize individual key presses. I have a JY-UL135N2 running the Feb. 22nd firmware update. I went into car settings / steering controls. It displays two rows of key functions that can be assigned to the SWC keys. I think you press and hold the SWC button and tap and the corresponding button on the settings screen that you wish to map to it. The top row of buttons in car settings have a number above each button which fluctuates between 254~255. The second row of buttons in car settings doesn't have the number above them, so not quite sure about those yet...

So it would appear Key 1 & 2 are ADC inputs that are measured in reference to ground, and the head unit sees a number from 0-255 depending on the resistance of the key pressed.

It looks like this should be a fairly simply project. I just need to find the time to do it. :)

edit:add - If you google "joying key 1 2 resistance" the carjoying forum thread about SWC is the first link (for me anyway), it's titled "Learning Steering wheel control for Joying car stereo - Joying Forum".