having a vehicle spawn ambiently in traffic with specific tuning parts is controlled by 2 things, 1) entry in popgroups.ymt and 2) for that model entries on which tuning parts / liveries in vehiclemodelsets.meta
Yeah, sounds easy, I tested it but never worked.
Moreover, even if it worked, it still spawns most of normal cars, just some are custom and only in specific area (parking LS Customs for example).
Finding a solution for most of car mods still doesn't make sense because they only have the basics parts like a custom spoilers, bumpers or skirts. However in very extensive mods, like my Lanos and Polonez it makes sense, because they use custom parts also for different bodies, so it should be option to set a lot of customs. For example for 3-4 versions of Polonez Atu (sedan), 3-4 of Kombi, 3-4 of Caro (hatch), 3-4 of Van, cabrio, coupe etc.
The same for Lanos.
For this moment can spawn ready combinations version/body only via Menyoo.
Thats why I told that mod-script for traffic with ini/xml file for configuration would be sweet...