I'm thinking that there is a short in a ground. When my turn signals flash my voltage-gauge drops a good mark and the headlights drop it too. Rolling down the windows and turning the heat to max drops it too. The headlights dim when anything else draws from the battery too. Could there be a short in a ground or bad alternator or what? The battery is a 1000 cranking amp and it's a lot bigger than the previous battery so I would think I wouldn't be having this problem.

 

Rolling down the windows and the turns can show a little voltage drop, that is normal. But if it major drop I would suspect the alternator is not working properly. You can take it to most autoparts stores to have it tested for free.

 

I tried doing that awhile back, they plugged into a port on the passenger side of the truck, it didn't give out any reading, then they tried a similar plug on the drivers side only to find it didn't transmit any signal either. Whether the truck was on, off, or in axillary.