Mercedesshop.com is doing a great service to all of us with this website. I just found it myself and have not since had a need to order parts, but I'm sure their competitive. Somehow it doesn't seem very fair to them to get all the tech support from them and their members, and then use their media to solicit "best price" information.
As for your climate control, what model are you working on? On the 124 and later cars, large and small, the symptoms you describe do indicate that the amplifier is very possibly the culprit. Follow the instructions that come with the amplifier and check the resistance in the hot water valve and auxiliary circulation pump circuits to prevent damage to the new amplifier, and I'm sure that Mercedesshop can provide you the technical assistance as far as resistance measurement values for use with an ohmmeter. Problems with the hotwater valve and/or the auxiliary circulation pump can cause damage to the new unit. Be dilligent with your tests.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in