Andy: 700 quid might well be a good enough figure to play with for a garage to do it maybe more depending on whos doing it.
Warren: I know exactly what you mean regarding the full looms however certain options are looked at by certain trends of previous buying patterns. For example if cruise control was fitted to say 30% of previous models then it would be highly possible that they would include the parts for the loom in all cars and push that option. However if its only 10% then its not in their best interest to include wiring (that can be installed separately in minimal time) to all cars that they do not have much faith that they will be able to flog enough to be worthwhile.
Take into consideration a simple thing like a 3rd brake light, it would not have been much of a problem to run the wires to the back shelf ready and waiting for someone to select the option and then just put the light in and change the shelf. However this is not the case and to wire in a 3rd brake light on an E34 requires you to remove the complete back seat and bolster, all trim panels down the right side of the car and to cut out the shelf and slot in a pre formed part of the shelf and bolt it through the boot. All this hassle to retrofit something that they should have put in as standard on these type of cars.
With that in mind things like aircon, cruise control, electric seats, electric blinds and many more options are things that they deal with on a per order basis. The OBC is a different situation because those that do not have it will have a clock in place using the same cables so retrofitting this afterwards is not as much of a hassle.
In regards to you finding the cable for the outside temp i would advise you taking the front bumper off and then look in the recess of the front where the water bottle is on the passenger side and you will find a little cable in there or it maybe part of the light harness along with the fog light cables for those that do not have them fitted.
As you can see from the picture above behind the bumper iron is a little yellow bit this is the outside temp sensor on my car so it may help for you to look in this general vicinity.
hope this helps
Russ