someone help me with my slant 6 problem

I've been having the same problem with overheating. One of the problems was with the thermostat; it refused to open once installed in the car. At first, I thought it was a faulty one, so i bought a different one. This time I tested it in boiling water. Opened up when it was suppose to until I put it in the car. Once in the car, nothing. And yes, I was putting it in the right way. I finally gave up on trying to make sense of it and tapped the center of the thermostat out. Tomorrow, I'm taking the car to work with me so when I punch out, I'm going to flush the system because there is rust in the antifreeze.