I agree with Dave. In my experience a seized accessory will just burn a belt up, but definitely make sure you can turn the motor over if you haven't already. I'm sure you're getting frustrated with having your starter tested so much, but really if you give it 12v, short it, and it does anything BESIDES slide the gear and turn like hell then it's not working effectively. The clicking means it's engaging, but not turning over. Causes for this are weak battery (everyone has heard that horrible click at some point), locked up motor, as Dave said accessory, or something jammed/lodged/wedged somewhere keeping it from turning. I'd have put a breaker bar on that bitch and cranked her over a couple times to see what's up. And make sure you go the right direction, if it only turns backwards you have a whole new set of issues! Haha.