If you are going to attempt this, get a manual of some sort, say, a Haynes manual. I have replaced heater cores but did not want to fool with either Corsica or Beratta (when we had one) as there is just not much room(plus both failed in the dead of winter, go figure). If you do some searching I have seen illustrations online, but been awhile. I will look around and see what I can find.
Here is one example I found... Berattas and Corsicas are similar, use the SAME service manuals and most all parts.
http://www.bstuff.com/index.php?showtopic=15029
