This is a little bit of kit I got to talk to the Webasto TT-V I salvaged from a scrap Jaguar S-Type, and converts USB-RS232 to the standard car diagnostic ODB connector. (These are a much cheaper option at £4 than the official Webasto diagnostic adaptor & loom which is over £90.
There’s really not much to this adaptor, the only signals that are routed to the ODB connector seem to be the +12v on pin 16, K-Line on Pin 7 & L-Line on pin 15. The main IC here is a CH340 USB-Serial interface, with some glue logic in the form of an LM339 quad comparator.
The reverse side of the PCB only has the power indicator LED.