I will further add what I have found in my data repository thus far.
I have a 2002 TrailBlazer and a 2005 Yukon. The Yukon has heated mirrors and the TrailBlazer does not.
In the Yukon the HVAC module sends the instruction to turn the heaters on or off in a message to the mirrors subsystem, functional address pair C2 & C3. Each door module responds to the message reporting each has performed the instructed action. The messages are highlighted here. The third byte is the senders address, 98 is the HVAC module, A1 is passenger door, A0 is drivers door.
I did not find the same for my TrailBlazer but this could be because I just haven't caught the whole operation yet in my data repository which currently consists of over a thousand recordings. That plus this TrailBlazer did not come with that option. I DID however find messages from the HVAC module instructing the mirror subsystem to turn off the heaters but NO instructions to turn them ON, and NO door modules respond to these messages. I intend to do a deliberate test for these actions today and will update here.