[mik]

the heater tap is under the inlet manifold on the six cylinders closer to the lower part of the engine from memory, sorry i have`nt taken much notice where they are on the 8`s if that`s what you have, defineatly worth checking for correct operation of the heater tap imo, if the tap is not closing you will NEVER have cold aircon, i`m 99% sure when there`s no vacuum the default position for the heater tap is fully open, good luck.