I am trying to find out how the firewall control for the heater works internally. Has control under dash that works by a rod. Does any one have an exploded view of this copper piece that come thru the firewall and the hoses connect to it. Notice that as soon as I open a window, it appears that the pressure level allows air to pass thru the heater box and into the truck cab. All hoses are really hot, so I do not think the control is stopping flow. Have thought about another valve in the hose line but the engine bay is really dress up for show truck look. Thanks for any help. Tom Duffy Cadillac, Mi.