These days, most of steering wheel remote controller is resistance-output.

 

Resistance-output steering wheel remote controls has 2 wire.

One is GND(ground), one is DATA.

Some times the GND is omitted, because GND is common for all the electric circuit of car.

Usually iron car body is GND and black wire of headunit's connector is GND.

 

Sometimes resistance-output remote controller has more than 2 wires.

RC-A, RC-B, and GND

For example,

RC-A is from left side remote controller

RC-B is from right side remote controller.

 

In this case, tie RC-A and RC-B and connect it to DATA wire of CarPC JoyCon.

 

If your remote controller is none resistance-output.

You can modify it into resistance-output.

Reference this post

http://www.rcjoycon.com/base_3/menu_11.php?com_board_basic=read_form&com_board_idx=2&menu=11&&com_board_search_code=&com_board_search_value1=&com_board_search_value2=&com_board_page=&