My 99 Mustang six, engine rebuilt at 200,000, now about 216000, a/c cuts off and goes to defrost occasionally on a grade - read this was either a cut-off valve that I bought - Motorcraft YG-360 - can't find where this is supposed to be located - I thought it was near the firewall. If this is not it, I read it is a problem under the dash with the vacuum control for the temperature control doors. Where is this YG-360 cutoff valve located on my 99 Mustang 3.8, auto? BTW - this has always been a great car - if I hadn't tried to limp home with a bandaged radiator hose, I would not have had to rebuild the engine -