Why does my headlights and my instrument panel flicker when driving at night time

You have a short,somewhere in you wiring, a lot of times, it's a ground, off of your wire harness, Gunar, just take a min. and open the hood, and have a look, or a wire under, your dash,that goes to ground.....hope this helps

I think I have the same issue on my 98 GT. Interior and dash, headlights pulse, this occurs at random, sometimes with check engine light popping on. Battery is new, connections are solid, clean, no corrosion. 80K on the 4.6L v8. Also, there is a pulsing squeak from the alternator that increases with engine speed. The car has monsoon 200+ watt factory stereo which is rarely on, I imagine I need a new alternator, as the regulator is integral?

With my 2007 Mustang V6 it turned out to be the positive battery cable.