It should be displayed in the head unit all of the time and to my knowledge it is NOT able to be selected / de-selected.
The outside temp display was introduced as part of the "Comfort 2" update package.
On a 70 Reg car, this should already have the C2 update already done ?.
Has the still car got all the annoying "Bongs".
This may indicate the C2 update has not been done.
Can you see the estimated range of the car shown in the main dash panel of the car ?.
This was a feature of the C2 update also.
If you only have SOME of the C2 updates, but are missing the outside temp display, then it needs the C2 update reapplying I think.