The main issue is a second <id> in car slot data that needs to match an existing id in carslots.xml. This may happen automatically if you import / reimport the carslots file with the server editor (1.6 beta), but I've been directly updating the carslots.xml file under persona 22418201 and not reimporting. So first buy a car just to get a slot that you will replace with the new car data.
Then copy the second <id> ... </id> data for the car you want to replace to save it, probably just copy it to the single car data.
Attached is an example txt file for GT 500 Super Snake The Run version. It includes some comments and the xml tags for the car.






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