If someone wanted additional heat...

[1986F150Six]

While searching for NOS parts, this popped up. It is said to be for a van, but this heater core looks like it could be located below or behind a bench seat.

https://www.ebay.com/itm/NOS-OEM-Ford-1983-1989-Econoline-Van-Rear-Heater-Core-1984-1985-1986-1987-1988/141926215203?hash=item210b762e23:g:8boAAOSwll1W1MnX