2003 Cadillac CTS rack and pinion

Steering problem
2003 Cadillac CTS 6 cyl Wheel Drive Type unknown Automatic 108000 miles

I bought a rack and pinion from autozone which caused all my power steering fluid to drain, which caused my belt to slip off, which caused my water pump to stop working, which caused my engine to seize up. After I replaced the engine, the mechanic said the rack was bad, and needed to be replaced also. He replaced it with another rack from autozone, and it still didn't work. I then got a rack from cadillac and it now works fine. Autozone is saying that their rack is compatable to my car. I believe otherwise. How can I prove their rack doesn't work for my car?