The electrical kit is purchased seperatley from the towbar anyway, depending on whether you need single or double electrics, I think subaru wanted £30 for the electrics, Halfords a similar price and the guy who fitted mine £10.
IIRC there was only 1 towbar from witter for all imprezas, but a genuine one may be different, I'd call a dealer to check.

 

Does anyone know if a towbar from a MY98 Impreza will fit a MY96 model???

I would think that the mechanical installation would be the same, but the electrical connection may be different.

Stuart