Converting a 12v signal to a J1850 VPW Serial Data

I apologize for being a total noob. I'm more of a gear head than anything but I'm looking for a solution to a problem. I am installing an engine management computer from a 2006 GM vehicle into a classic car. I have just about everything working like I want with the exception of one thing. In 2006, the ECM is looking for serial data when the original Air Conditioning compressor button is turned on. That is being provided by the 2006 control head for the HVAC. In my case, I can't provide that signal because I'm using an aftermarket control head that just provides 12v.

Is there a way to make a little converter that will take the 12v power when applied and then output the serial data that the ECM is looking for? I know this has been done on a slightly newer ECM that runs CANBUS, but I can't find anyone who has made this work with the J1850 VPW Serial Data.

Thanks in advance.