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No best answer has yet been selected by nicademus. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.Earth is negative so just put the negative wire to a suitable earth point. If you are making a circuit then ensure you fuse the live wire!!!!
You can either make a permanent live wire or an ignition live (wire is negative with ignition off, but once ignition switched on it becomes live). It obviously depends on what you need the wiring for.
Surely if it's a plastic car then it doesn't have an earth as such, all the electrical devices have to have a return, but not through the earth.
To the original question, what circuit are you trying to fit, to which car? More to the point, I would respectfully suggest that if you are not 100% sure of what you are doing, either get it done professionally, or better still, get somebody to do it whilst you watch and learn.
Scrapyards are full of cars that have been burned out by dodgy wiring.
If you are going to do it yourself, assuming the -ve on the battery goes to the body of the car, then you can either wire the return to the body, having first scraped off paint, or if it is a large current device, then you are better going back to the terminal itself.
Hope this helps