Chevy Astro heater fan

Hi, David here, this is my first post.
I'm in the Midwest and have an 03 Chevy Astro LS with AWD that I've just purchased, with a lot of highway miles.
It's getting me thru the snow just fine, but there's a problem.
The on-dash heater switch worked OK at first.
Then the next week on the first three positions, but only occassionally on high.
A few nights ago, it wouldn't come on at all, then a couple miles down the road, the fan came on and we got comfortable.
This week, no heat fan at all!
If I can't get this going, I'm not going to have windshield defrost, much less heat for my toes.
Do I need to tear into the dash or what?