I'm still interested as a back up plan to help all mylink users interested in making their car safer if the GM calibration number mentioned above turns out to not be feasible. I suspect your solution isn't Spark EV specific and would work on a *lot* of GM vehicles that use this same head unit but weren't equipped with a factory backup camera. This seems like a great thing to be open sourced so people can re-create, and maybe with enough momentum somebody will make a plug and play kit out of that design.
I've been interested in trying out a low volume run of custom PCBs and components, there's a place that specializes in this nearby me called Screaming Circuits:
If you posted a schematic, parts list, and your source code, this would be 90% of the info needed to recreate this without a ton of effort on your part. It's hard to tell in the photo, but it looks like with a small, custom-made (and populated) circuit board, the pre-made Teensy board, and a couple of headers, no soldering would be required at all.
Personally, I'm pretty good at sourcing parts. I'm sure I could easily come up with a parts kit for the car that would make the harness part plug-and-play. I'm also good with CAD if that helps, either turning a schematic and parts list into a PCB layout or creating an enclosure with integrated connectors. I'm pretty bad with software and electronics design, so I'd hope somebody else could help come up with some LED indicator circuits and instructions on code stuff.
I still hope the GM calibration would make this all avoidable, but until we can try it who knows how it might work out...