A piece of 0000 grit steel wool, some mildly abrasive stuff (toothpaste works) like noxon or bon ami and rub it in a circular motion directly in front of the bulb. You don't have to bother with the rest of the lens, really.
Be patient & do it right.
When done, liberally apply a top quality paste wax to the lens.
Weekly, I wipe them with a thin coating of light mach. or olive oil. The oil also makes your chrome gleam
Jim in Quincy MA, August 2006