Make sure your battery cables are good and snug, that also will cause the condition.

 

Mine did this too, except the interior light did it the most. Took the alt. off and had it tested, it tested good but I replaced it any way and everything stopped, no more flicker. I'm not saying this is your problem but it was certainly mine.

 

I replaced the brushes in my alternator and the pulsing went away. they were worn badly. might check that as well. Cheap and easy fix.