Finish your HSC mate. Yeah you may want to be working on cars now, but what about in 20 years time. Even though people dont look at your marks when you have been out of school for a while, but people still like to see if you have completed you HSC. think about it. Its only 1 or 2 years out of your life and once you have finished you can do what ever you want, whether it be a mechanic, do a engineering degree. The world is your oyster mate, think about the future and sometimes you need to put present emotions aside as they can sometimes blind you.
What ever you decide mate, good luck and have no regrets.
Ben