Speakers, front and rear, except the twitters, quit working

I have had problems with the speakers in my 2007 Avalanche for awhile. The front drivers side are the only ones that worked, except on rare occasions the passenger side would come on for brief periods. I always keep the truck garaged but last week I had to have it outside for a week. We had a lot of snow and rain and when I drove it all the main speakers did not work, only the twitters.

What do you think it could be and what would be the cost to solve the problem?