The CANBUS has been found to go bad at the splices because of corrosion. They are hard to locate. If you test at the PTC as in the referenced post you will know if it is bad or good. Don't go hunting until you have tested at the PTCM connection. All the CANBUS enters the PTCM at just that one connector, so it is easy to test entire system at the one point.
Good to see you have the wiring diagrams.
The Nag1 transmissions is the same the same as Mercedes 722.6 and so you can scan for Mercedes specific codes by entering 2003 Mercedes SLK 320. It is a R170 chassis. If the car was supercharged like the SRT6 you would scan as a 2003 Mercedes SLK 32.
If i can download a picture this info may help:

GOOD LUCK