I have recently partly fitted a 2nd hand Witter tow bar. It was not immediately apparent exactly how it went together, even with the fitting instructions that I downloaded from the Witter site. That said it was not difficult. I have still have the elecs to fit and the silencer gets in the way of one fitting and will have to removed temporally to fit. The elecs do not need a relay to fit, so that does make life easier.
The bumper is a tight fit when the bar is fitted, but not impossible. Just take the bumper off and start fitting, if it takes a few days, don't worry unless you reverse into someone.
Good luck