Check your battery cables and your grounds, not just the big fat one, but the little ones too. Check for a grounding strap from the engine to the frame. Often the lights, and speakers will ground to the body.
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BroncoJoe19
2006 Alt Fuel Jeep (electric)
'98 windstar 3.8L
'99 Crown Vic 4.6L
'90 Full Size Bronco 5.0L
I'm not a professional mechanic, take my suggestions with a grain of salt, or a cup of coffee 
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