If you want it to work... then ground it to the body...
If you are asking where it is grounded on a stock car, sorry, can't help you. Electrically, it makes bugger all difference where it is grounded. The cable runs cannot possibly get long enough for cable resistance to reach a point where the earth won't work properly.
Just ground it to which ever ground point is easiest for you to run the cable to, keeping in mind that making things neat and tidy is better in the long run, even if it means more work now.
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