Changed my search criteria a bit, and found an Amazon vendor that carries a Crown aftermarket cap, for about half of what Mopar wants.
I suppose there could be something wrong with the "splines" in the new parts - we don't know that they were made right. But, I've had trouble getting those splines to seat on some vehicles. Can you get to the nut without opening the hood? You may have already tried this but one thing that sometimes helps is to pull the arm up and away from the glass and "lock" it in the "away" position so there is no tension from the spring being transmitted to the splines. Now you can rock and wiggle the arm carefully as you tighten the nut. As you rock the arm the cone-and-spline mating surfaces might settle better and you might end up with a tighter joint. You might even be able to "feel" your way into the splines as you wiggle the arm and snug the nut at the same time. Of course, the way my luck works, once I got it tight I would find that I was off a spline or two and have to start over
Well, that's the funny thing. Replacement OEM and Dorman arms have a reamed or machined out aluminum arm that's painted, but no splines at all. It's pretty apparent that the harder steel is intended to engage with the softer paint and aluminum.
You know, I had wondered about that. All the ones I've worked on (except a couple) have been on older vehicles and big trucks. The old Mopars had serious, full length splines which can be a pain to get off. I have used my wiggle trick mostly on old military trucks (which have a very small and slightly conical spline area) and on my Wife's newer BMW. The BMW wouldn't stay tight until I did the wiggle trick and "inched up" on the nut a little at a time. And that car is a bugger because you can not pull the wiper up off the glass very far - the arm will hit the edge of the hood (terrible design).
