It is a super-good idea to make sure one's wipers run and park in the correct spot before installing the unit in the car! Ask me how I know.
Having said that, it was possible - just - to loosen two of the three clamping screws enough to rotate the cap with the wiper installed in the car.
I had my wipers rebuilt by one of the usual suppliers (no names) and he re-assembled the unit without testing it. First, it didn't run at all, then the park was in the wrong place. I fixed the former on the bench - the brush unit had fallen off to one side inside on assembly - fortunately I found a YouTube video explaining this. Second, the park was in the wrong place and I had to fix that in the car after I'd installed the wiring harness and gauges.