Try the Kitchen sink. Make it into something fun . You could use a changing mat for her to lie down on and roll a towel for her head to lie on and gently wash her hair in the sink . You could give her a toy or book or something to do whilst you wash her hair . X
We bought my little boy a toy shower head, which helped. It's one that you press yourself and it pumps up the water from the bath. It gave him the control of being able to pump the shower head and aim it himself. X
Same here. We wash it very rarely. Like once every few weeks.