You can access the CAN bus pretty cheaply (there are several cards available around $200 with linux drivers, maybe cheaper if you're willing to do more than plug it in and turn on wireshark). That said, the last time I worked on ECMs, the messages were not standardized at all. You needed a spec to define what variations of ECMs were available as well as what the individual values would mean. I feel like GM was standardizing this more than others in an effort to consolidate ECMs they were shipping (runtime config via "calibrations" I believe), and all manufacturers might be using standardized parts now, but you'll still need to know not only the message inventory for your vehicle, but you'll want to know whatever diagnostic protocol that your manufacturer uses to sit on top of CAN. Keyword Protocol 2000 was used by DaimlerChrysler was used when I was still in the business and GMLAN was used by GM for these purposes.
The Goodthopter [1], an offshoot of Travis Goodspeed's 'GoodFET' project, allows CAN access for about $35 all in. It's basically a MSP430 + a CAN controller.
Even 'simple' CAN protocols, like J1939 used in trucks/agri/marine, are purposely non-standard. It's rather annoying.
I'm curious as to what systems will emerge from Mercedes/VW/BMW (and I guess Google) as more high speed sources like LiDAR, active dampening, etc become the norm. The current BMW 5/7 series and Mercedes' S-class already have separate CAN-like systems for high speed buses.
