I think in most cases it is a base or mount, an arm and a spring that puts tension on the arm.
In the first failure I had, the issue was that the arm seized to the mount and wouldn't move to maintain tension on the belt. This allowed the belt to slip off and get eaten.
In this case, the arm is no longer parallel to the engine. Not really sure what the failure is, and it hasn't caused any problems so far but pulling a hill at 3000 RPM is not where I want to have one either. I've got a squeak, too. The YT video in the other thread I link above made mention of the tensioner squeaking, not sure if that is the source of my squeak or not.
Here's a picture of the idler from earlier in the thread. The '91 part is the dirty one on the left, the other one is a '92+ Magnum tensioner.
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