There's a little more to replacing the thermostat on the 4.2-liter engine in your 2003 Chevrolet Trailblazer than just popping off a radiator hose and removing the thermostat housing. I found that I had to take the alternator off to get to the thermostat housing. Now that I've done it once, I can share my experience with you so you don't have to do as much finagling as I did. When you need to lift the front, use the jacking point on the center of the front crossmember and put the jack stands under the frame behind the front wheels.
Getting to the Thermostat Housing
You have to remove the alternator; but to remove the alternator, you have to remove the engine lift hook. That's not a big deal. After you disconnect the negative battery cable, remove the bolt holding the air conditioning line bracket to the hook and then remove the two bolts that hold the hook to the engine. Now you can get to the alternator. Use a 3/8-inch breaker bar to rotate the belt tensioner and then slip the belt off the alternator. Three bolts hold the alternator on. Take them off and take off the nut that holds the positive battery wire to the alternator. Now you can get to the thermostat housing.
Button It All Up
Make sure you tighten everything on reassembly. Tighten the thermostat housing bolts to 89 inch-pounds, the positive wire nut to 80 inch-pounds, the alternator mounting bolts and engine lift hook bolts to 37 foot-pounds. Tighten the air conditioning line bracket bolt to 89 inch-pounds. Refill and burp the system, using a 50 percent solution of Dex-Cool antifreeze and clean water.