When I bought my car recently, I found exactly that problem, that the little post that the wheel rotates on was snapped off. I did a homemade repair this way:
I drilled a 1/8" hole down into the centre of what was left of the post. There was about 3/4" of plastic to drill into. I had to experiment a couple of times to get the height just right so that the wheel could sit on the nail of a pop rivet that I inserted into the 1/8" hole. To get the height right, I had to drill with a 3/8" drill to get the top of the boss to be a bit lower, and then I used the pop-rivet tool to start the nail into the rivet, but not enough to snap it. I cut the nail off with sidecutters, at a height that would fit inside the installed cover. I created a couple of bushings with some 1/8" copper line ferrules for compression fittings that fit into the axle hole, on the top and bottom of the wheel. The cover was cracked and a piece of it had detached so I glued that back into place and backed it up with some black duct tape.
When I put it all back together, I turned the wipers on and the whole thing clicked and clacked a couple of times, and to my surprise, the wipers work perfectly. So, I'm going to leave it like that until I actually find a cover that is going to work, but without paying a stupid price for it.
Have fun!
