Check for bad earth, here's how:
You need to do a 'voltage drop' test to the battery negative from the engine to the battery, the engine to the frame, the frame to the body and the body to the engine. To run this test, you need a voltmeter that will read accurately to tenths of a volt. The readings are taken by putting the 'black test lead on the battery negative post' and touching the 'red lead' to the engine, frame and body. The reading should not read over 'one tenth of one volt' with the engine running and the lights turned on. Any reading higher than this will indicate a 'bad earth'
If you haven't got a digital multimeter, buy one, you will always be using it if you do your own repairs.. You can get a good cheap one for around $20.
"I married Miss Right. I just didn't know her first name was Always."