It would take me several pages to list the dumb-donkey stunts I've pulled since I started getting paid to fix things in 1958- - - -it's all part of the learning curve, and the embarrassing and/or expensive ones really imprint their lessons effectively. Since the lights brighten up with voltage going directly to them, start looking for a loss of voltage from one part of the circuit to another, I like to use a voltmeter with a LONG ground lead attached right to the battery's grounded post and start measuring voltages everywhere I can along the circuit. You're going to find that somewhere along the way the voltage will take an unusually big dip, like maybe from the supply wire for the light switch to the terminal that sends voltage to the dimmer switch, for instance. That would indicate a high-resistance (dirty) connection inside the switch, and/or a loose, dirty connection at one of the terminals. Another dead giveaway of a problem is a switch, a wire, or a connection that is warm to the touch- - - - -resistance generates heat. Obviously, all these tests need to be made with the lights turned on. Nothing measureable voltage-wise happens in a circuit when current isn't flowing. Good luck!
Jerry
