Right behind the engine.
:) Sorry, couldn't help myself!
They're mostly found in late '80s to mid '90s vans and light trucks (2wd Daks and D100/150s) with the V6 and 318 engines, although I've heard of them showing up in some 5th Avenues and such- but I can't confirm.
Mid production they changed the designation from A999 to 32RH, same transmission though.
