I agree and sometimes the battery ground cable on the battery post connector (if it is the crimped type) can go intermittant. Sometimes it will make up (connect) with a heavy current draw but have a high restance under light load. with the lights on dim, feel for any heat on the clamp or just make a temporary jumper from a good known ground to the batterry negative post. You might be able to touch the top of the battery post with the lights on and observe for an increase in brightness. Another clue would be any white residue around the crimped connection. Also a voltmeter test between the engine block and the battery negative terminal should show zero with a good ground cable.