I'll crack the schematis and see what I'm dealing with. What are your thoughts on the pedal switch? I thought of the same, but it is only on heavier braking, not all the time.

In theory, there should be no correlation between the headlights and the braking light circuit.

With the brake light switch, it is also a simple device - open or closed. It is not variable, so the amount of braking effort shouldn't bother it. Otherwise (for example) sitting in your driveway with your foot mashed to the floor on the brake pedal could produce the flicker.

My thoughts are now gravitating towards it being a crook ballast.

Ballasts interchangeable? :)

Honestly, I have absolutely no freakin' idea. Certainly worth looking at the connectors though. Ballasts are generally not specific to one side.

You could swap the ballasts side to side, if the flicker follows the ballast then you kbow