mopar 1/20/2012wiggle your head lamp switch..gets bright and dim?....replace switch
rick 1/21/2012How old is your altanater ? The voltage regulater is built in and most likely going bad . When it kicks in and charges the lights are bright . Then it stops charging and everything is running on the battery only . So all the lights go dim . Sounds like time for a new one before dead on side of road . Take it off and most auto parts stores can check it for you for free . Make a call or two some can check it why still in your truck . That way you got something to drive to pick up a new one .
mike 1/21/2012if your previous answers not helping check alternator belt for slippage or alternator itself. if under load (radio, blower , GPS ) could possibly be battery