There needs to be a positive ground either to the frame or engine. Grounding the main battery feed through the sheetmetal is NO GOOD. The sheetmetal can't handle that amount of load. If you want to ground the body directly off the battery that is fine but you need 2 cables, 1 for the body and 1 for either the engine or frame. You can ground the engine through the frame or vice-versa if you want.
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Dennis
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