I suspect that the hazard lights were already on before they reached the carpark, and the wipers were switched on by mistake, by the killer, during the murder.
And not only that they also managed to find a free space in a relatively obscure part of the car park that was partially shielded by overgrown hedges. We don't know if that car park was specifically chosen or just a randomly chosen place. It does seem quite remarkable that such a frenzied attack could be carried out without being noticed. Perhaps initially something else was going on and then the killer just lost control of themselves. I do tend to agree that forcing someone to drive for a significant distance is no easy task either.
the space where car was parked up was hidden from view by the bushes, so iam not surprised no one noticed the attack.