I monitor the Tacoma simulcast site as it is a strong signal from my location in south King County. I receive Tacoma WSP east and west, Seattle South King, North King, Metro, and sometimes Olympia and Academy.
I noticed that they were calling 59203 Car 2 some time ago. I changed it in my database from Car 3 to Car 2.
I assumed that Car 1 (which I haven't heard on Tacoma simulcast) would be for District 1 Pierce County and Car 2 would be used for District 2 King County etc..
I also asusmed that the different districts have Car 1, Car 2, and Car 3 to talk car to car from one district to another.
This doesn't explain why 59203 is Car 2 though. It doesn't follow the convention of the last digit of the talkgroup relates to the channel name, and the third digit is the district. This is of course assuming the database is correct and not assumptive.