You did not mention what model you're having trouble with. On my 300E the amplifier is contained within the pushbutton unit. A sheet comes with the unit which gives you resistance readings at various pinouts for testing the system. You can make these tests with an ohmmeter, you don't need specialized MB equipment.
I would expect that MBShop would provide this sheet to you as technical support. If they are kind enough to provide a copy of the troubleshooting sheet that comes with the unit for you to troubleshoot with, it would be fair to purchase the unit from them should you need a new one.
On the 300E, you remove the woodtrim panel from around the pushbutton unit, then remove the pushbutton unit. Disconnect the connectors to the unit. Use an ohmmeter at these connectors to test everything going to the unit, especially the auxiliary pump and the hotwater valve. If they test okay, replace the unit.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in