Is it possible to convert manual single zone air con 3 dials to manual air con dual zone 4 dials. My car has room for the extra dial and the manual says it should have manual dual zone i.e 4 dials. Was a late production car. IS it just a matter of making a connection with extra cable. Is a new heater required or a distribution box. Any advice would be much appreciated as no one seems to know the answer.