Mark,
Only thing I could think of is maybe find a long straw or something and tape it to where the arm is supposed to attach and turn it on and try and mark the limits of the swing to left and right and get an approximate length and then pick up the parts @ an auto parts store?
Only other idea I had was maybe trailer her to the jiffy lube- they always seem to want to add a filter and new wiper blades to my bill when I go there in my truck!