Please, please, please remember this is just a temporary repair/test not to be taken down the road. There are wires already there somewhere mounted to the frame by a stud or more likely bolt and nut or screw that is loose or rusted or broken. If this works then you need to find, clean and repair them properly. Wire nuts are not the way. Crimped or soldered terminals with heat shrink and conductive anti-corrosion coating should be used to properly make permanent repairs. More than likely if you hook up a ground incorrectly to a positive you will blow a fuse. Going down the road not knowing where the fuse box is is just asking for a huge towing/repair bill. Follow the harness, look under the dash and find the fuse box.