Thanks all,
I removed the A/C fuse from the fuse box and tested it with a multimeter. It's OK.
The wireless is a Kenwood KDC BT21 as in attached pics.
Will head off to SuperCheap and buy a tester. (Going to buy Stick-on Rear view Mirror as well from them).
After Brekky I'll go through the entire set of fuses under the bonnet.
I do appreciate all your help as the car has been giving us a new prob each day - starting on Christmas Eve !!
Well, we had the sun roof cocked up to let hot air out - and last night - it wouldn't close!
The 'close' button felt as useless as the air-con controls!
Just as an aside.