What I'm finding from searching is that on a commercial Chevy van of that vintage, not equipped with a factory radio, which means ours, there was no ACC wire conveniently available.
The aftermarket sells an all-inclusive multi-purpose wire harness which connects into some main wiring junction under the dash and does include an ACC wire then. But it's $200 because of its complexity. So forget that.
A kludgy workaround is to go underhood and piggyback off the wipers fuse. Apparently they work on ACC.