Flickering headlights and interior lights
 

I replaced the alternator last December with a brand new one due to a bad voltage regulator causing flickering lights on cold starts. Alternator seemed to fix the problem. Battery was replaced in March of 2015. Drove across country and back no problems. Couple months ago flickering light issue came back. Took to the alternator shop they tested alternator and battery as good. Last Friday my truck left me stranded with less than 9 volts. Jumped it off drove to auto parts store where I bought the battery and it tested bad at 425 CCA even though it was rated at 690. Took it out put on a charger but the battery was bad. Replaced under warranty. Truck cranks well and runs fine but on cold start it has flickering lights. I was thinking headlight switch may be the culprit. Any input would be appreciated.

It would be helpful to know the year, make and model of your vehicle. I see in your "garage" you have 2016 Camaro listed.

Flickering lights
 

That's odd as I have both my vehicles in the garage listed. 2003 Chevy blazer ls 4x4 is what I'm having the light flickering issue with.

Check the voltage with a DMM at the alt when the lights are flickering.
Some vehicles have the voltage regulator set up like that to give power to more important things while the vehicle is warming up.
I know my Trailblazer is set up like that.