This was originally by Chronus
There are cars, complete mods and just skins.
Cars: unpack them to a safe location and check which folders are created there. Then, you should be able to copy: TEAMS to TEAMS, Talent to Talent, SOUNDS to SOUNDS. check if these cars come with championship files.
Mods: the same as above, or run the executable (some mods are packed into an executable). Follow directions but, to be on the safe side, point to a safe location before you (as above) proceed to copy files/folders to GTR2 installation folder.
Skins: these are just skin folders - and normally go into TEAMS->SOMECLASS->SOMECAR.
SOMECLASS may be 24H, GT, NGT or other non-default classes or vehicle folders.
SOMECAR may be, for instance, MOSLER Teams, within which you will be pasting the skins folder(s).
If, even after following the above, cars are not detected or simply won't run, then there's something else. To be sure, any vehicle folder should have GTR files, hdc files, tyr files (or simply have hdc point to default ones), eng files, aud files, sp files and grb files. And, last but not least, cas files. Inside skins folders (the ones coexisting with all these files) you should have car files (at least one) and some dds/GTR files (not absolutely required).
If not, no go.
More, you need to check some entries in the car file:
You need to make sure that gdb files for the vehicles (files at the level of gamedata folder) contain entries related to the vehicles and similar to the items I mentioned above.
Sounds complicated, but it's not. Anyway, anything that's good is worth learning or fighting for.
Hope this helps.