1986-1993 VOES switch should have a ground wire with a ring terminal that terminates at the top center motor mount bolt to the rear head.
(1986 may have originally grounded to the rear of the center mount bracket but it's safer to use the bolt thru the head)
1994-2003 VOES switch has a ground wire in the same connector block as the signal wire and is grounded by the main electrical system grounds.