I'm not much help as I have only driven Bedfords but 18" to 24" sounds right for a conection under the dash. I do remember the Hi /Low in the Bedford was like a choke cable on the dash which made it fun to change box and diff at the same time. the switch tap for the vacuum was on the firewall under the bonnet and connected to the manifold. Nob out connected vacuum and pulled the diff into high, nob in no vacuum and a spring engaged low, so you were stuck in low if anything went wrong with the vacuum. Unfortunately I carn't remember what was under the dash.
