lights on -lights off
your switch is bad sending a short thru andd causing the flicker
you might as well tell him it's the wire, could you be any more vague, more than likely you have some dirty connections, ie battery, alt, or the fuse between battery and alt, (it's on the back side of the motor toward the firewall) clean them all up including the grounds and then go from there do the free and very little effort things first, im gonna take a stab in the dark, does your dome light dim with your heater on high and lights off?
Search Mercury Grand Marquis Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Mercury Grand Marquis Experts