Where can I find a diagram of a 1990 300sel 's heating and cooling system?

There are repair manuals on CD on Ebay. I bought one, but couldn't really make heads or tails of it. I have an 89 300SEL that didn't heat or cool. I recharged the AC and got it working (after tightening a loose hose), and got the heat to work after replacing the Mono Valve diaphragm. The AC work cost $30 and the valve repair kit was $65. I got it from PerformanceProducts.com. I hope this answers your question.