Maybe you tell us first what you have installed now.
No aircon? Just a heater? Or?
Anyway, the best and most detailed info is on a German site from Gunnar 525, he has described it all on about 60 pages with pics. Even if you do not understand German, you will learn a lot. See on the site on the left under "Schraubereien".
http://www.gunnar525.de/