Single wire Can GMLAN module. The ZIP contains the Gerber if you want to build one.
Here's an example code that lets you TX and RX GMLAN 33.3k. It's still a work in progress. TX needs a function; currently, it just sends a placeholder frame. It uses this library: https://github.com/autowp/arduino-mcp2515 I have not tested on the latest library release, but it should work.