BMW Repair: coolant/radio, coolant level, crackling noise

Question
hi,
i have a few questions, so thanks for your time :)!! i just recently
bought a 1989 525i executive. I'm a student so i really dont
have much money to be spending on repairs and would like to
do some repairs myself (if possible)..
Anyway here is my problem.. Every time i turn the car on or off a
check coolant level - owners handbook light comes on, but ive
checked the coolant level a couple of times and it's full and not
going down drastically.. only a bit over the last three weeks or
so... Just wondering if this is a sensor problem or something
electronic like that.. ? and (hopefuly) i can fix myself without too
much hassle...?

Secondly when i turn the volume control on my radio it makes a
crackling noise and but when i turn it very slightly it goes away...
but as soon as i adjust the volume again it happens... My
mechanic said that it is something inside the radio and he knows
how to fix it but once again im a student and want to get out of
paying for any repairs i can (that aren't too hard obviously) and i
want to get to know my car a little better and do some repairs
myself...

Lastly (although this is a stupid question) the Outside
temperature gauge that tells you the temperature outside (on
the same screen as the clock) is in Fahrenheit and (scene as im
from Australia) id like it to be in degrees Centigrade... and i dont
know how to change it... i had a look in the handbook but i cant
find where it says how.. i dont have an OBC... and i cant find any
switches that change it..?

Thanks very much for your time :)

Answer
First the coolant sensor is probably bad,its on top of the reservoir an easy fix ,replace it
The radio is dirty it needs to be removed and cleaned
And finally I thought you could just move a swith next to the clock it has km/miles on it.Moving it to km changes the function