It is the big purple wire I think some one may have done you the "favor" and bypassed the switch by twisting the two ends of the purple wire together eliminating the switch all together. The switch may have gone out at some point or lesser still gone out of adjustment and instead of fixing it right they fixed it for good. That happens on older stuff when people start getting tired of that ride or their money is low and they have to do whatever to ride. It does make for some cruddy repairs sometimes and also makes for dash fires and underhood fires and things like that as well
. Jim