Antenna Again

It's been less than 2 years since I replaced my antenna and just like that it's pooched again. If you've had to replace yours in the past you know that it's not cheap and the only place to get it is from Toyota. My question is what is it that has failed, I have not taken it out of the fender yet but I'm going to assume that the metal base that sandwiches to the top of the fender has rotted, oxidized. I've tested the ant with a piece of wire at the connection in the passenger pillar and right away I get signal. I grounded the outer shield and nothing happened so now I'm thinking the inner wire is the problem. I'm just trying to understand what has failed and why.