Ford F-150 - 1997-2003 :: 2001 - All The Lights Pulsate At A Steady Rate When Running

2001 F150 4x4 5.4 gasoline, 200k miles

I have had the vehicle for about 3 years now. One thing I noticed when originally I got it, was that all the lights, headlights, tail lights, dome light, instrument panel lights would pulsate at a steady rate when it was running. It did not matter whether it was idling or doing 70 on the highway.

When I bought the truck, the stock plastic headlights had long since seen better days. So shortly after I bought it, I swapped out the headlights with new high intensity ones. Each side has it's own high voltage power supply. They have worked fine since I installed them.

Tonight I left the truck running when I parked, and I got out and looked at all the lights. Headlights are fine and steady.

All the other lights, parking, instrument, tail, dome, are still pulsing. It is a steady constant pulsation of less than 1/2 second pulse. The only light that was not visibly pulsing was the digital display on the radio.

What could be the culprit here? Ignition noise? I have never replaced the alternator. It may be original. The battery is less than a year old. Again, it's done this pulsing since day 1, when I bought it used and BEFORE I replaced the stock headlights.