if u install now cars attribute plugin,and activate this for "auto" categories,go to edit a ad(from auto category),cars attribute appears but the data is not saved,because this plugins was installed after respective ad was posted!
i someone has problems with this plugin:
check if your "vehicle types " is not empty in admin panel
and check debug.log in your theme folder to see the errors and warnings
my problems was with my translated folder....someone translated this and upload folder ro_RO to forums...but translated folder was not valid!(was wrong format)
after i translated myself ,now works ok!
if someone has problem with this,im here to help!