For those of you who don't have all add-on cars, use the vanilla version of the popgroups.ymt and read 
the readme included in the said folder to add the add-on vehicles that you have.

NOTE: You can also use the popgroups.ymt from the with add-on cars/with add-on cars & race cars
and with manual installation vehicles. It included ALL the add-on vehicles, but don't worry, you can
erase the line for the add-on pack you didn't have. (e.g. if you don't want/have Civic SI you can erase
the line <Item><Name>civic</Name><Variations type="NULL"/></Item> from your popgroups.ymt)
Item name depends on the vehicle's internal name.