Wiper arms can be held on the splined shaft by different methods. I do not remember how the 31 Tiaras where done.
Some have a spring clip. Look or feel under the arm by the shaft and see if there is a little flat stainsess clip that can be pulled or pushed. It won't come off, it just retracts enough from a groove in the splined shaft to allow you to pull (you may have to wiggle while pulling) while you pull or push that little clip or spring.
Some have allen set screws to hold the wiper arm to the shaft. I have not seen this style in a long while and I believe the 31 LE uses the clip on version, but look for allen screws just in case.
If the arms are corroded to the shaft, you may have to spray some penetrating oil up under the arm and let it sit for a while. Place a rag around the area when spraying to keep the mess to a minumum.
Dremel tool works fine if you are cautious not to dig into the splined shaft hub too much. But before you go there, see if you can find an open ended box wrench to slide under there and pop the arm off.