53 Reg V8 petrol heating is working fine in the front of car ! But cant get any heat from rear blower unit , Fan is working but not giving any heat, is there a valve or switch under the centre arm rest that controls the heat ?

Not aware of a valve within the system as air is directed from the front.
The rear vent has two thumb wheels, one for speed & the other for temperature. Are these working ok?

Have you tried switching the front flaps to foot well only & see if that helps?.

Yes the fan speed works OK, its the temp one thats not working, there must be something that it opens & im not sure if it control'd electrically or by a cable, it should work independent of the front controls.may just be a fuse or a relay.

Daft question but when you check the back are you leaving the fronts on a heat setting? The rears cannot be hotter than the front. John
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Answer to that is yes but they used to work independent of each other, same as the front each side can give different temp thats why you have 2 x temp controls , the rear has a temp control and fan speed, So some sort of valve must turn heat on or off for the rear passengers,

There’s a button on the dash to enable control of the rears otherwise it just works to supplement the front temp.