Have a problem with my lights ,only when cold, when I turn on the heater fan my dash lights head lights everything electrical dim, but only for a few moments. Once the engine is warmed up = No problem. Alternator belt is tight, battery is holding a charge. No fuse issues. Just this problem when engine is cold.

 

Have you taken your battery to a local auto parts store for testing? AutoZone, and the like, will put your battery on a load bank for FREE that will test your battery for strength and endurance. Even though your battery may be providing 12v now, it sounds like your battery may be nearing the end of its days. You don't want to break down in the dead of winter for lack of an easy fix. (ask me how I know.) Get the battery tested. It will provide peace of mine and let you know how much life is left in your battery.