I worked on a Racing game before, that had
the rights to use a brand from a single manufactuer.
They are really picky about their car brands, and
did not even allow showing damages on the car, etc.
Its very hard to legally use the brands, and
almost impossible to get them for multiple car types.
(You need to be EA or Microsoft to finance that)
Best is to use your own creations, that mimic (not too close)
known carmodels.
Or, as suggested, try to contact a small manufacturer,
they might like the idea to have some free advertisement
for their cars this way. (Im thinking about Chinese, Russian or Indian manufactueres)
(Mercedes or Ferrari probably wont give you permission though ;))