Hy Apache_Jake, firstly your truck was 12 volt from the factory, as were all since the 55B series as far as I know. A certain amount of light intensity change from idle to driving rpm is completely normal as has been stated. A check with a voltmeter will verify your charging voltage, as has been stated. If you have already replaced the dimmer switch, then I would check the headlights to make sure the highbeam filaments are not burned out.
