Dave...if you don't think the crack is from a rock or some sort of damage (crash, bump, wild hammer swings in the garage), then it could be from your chassis flexing. Find out if people with the same car and cage as you have this problem. I would check all the welds on your cage and spot welds on the car to make sure you haven't broken anything. Having a window crack due to chassis twisting would be a sign of a bad thing...If a bunch of people with the car and cage have the problem, you might want to add a bar or bars somewhere to couter act the problem, if not, you might have a problem specific to your car.

It could also be from vibration, in which case you might be able to fix it by changing how you mount it, but rule out the most serious possible cause first.

Just my .02