Medium High speed just went out in my truck (The one I almost always use). Chances are its a wiring problem at the resistor strip, but I've never had to work on a resistor strip on one of these newer model vehicles. Its not near the firewall where I'm used to finding them. I suspect it may be under the dash somewhere, but I'd like to know exactly where to look before I started indiscriminately ripping the dash apart.
The newer S-10s have them on the firewall, mine has it buried on the motor. I'd assume since you have a "newer" Sierra, it'd be the same. If it's not on the firewall near the intake or AC exchanger, I'd assume its on the blower housing, buried under the dash. I only have the 2 fast settings on mine, just not worth the hassle of ripping the dash off to find the resistor.
Yeah, most people would just say oh well, and leave it with 1 speed not working, but I make a point that everything on my cars work. The only thing that does not work on any of my cars is the cruise control on the Buick, and that's because the resume solenoid is broken, and I cannot locate a replacement. Because of that, the car "resumes" the second you take your foot off the brake. I ended up disconnecting the cruise for obvious safety reasons until that part can be located.