Aircon Problem...

I have a '92 500 SL with an aircon problem. ie it doesn't work! Won't blow cold or hot, fans don't operate! Recently had the problem diagnosed as needing a "main control unit" but with no guarantees of a cure. Cost me about £350 for unit and fitting and, voila, it worked perfectly! About 3 weeks later, (yesterday), it stops working again I'm thinking that maybe there's some other fault which has maybe caused the new control unit to blow. Don't want the expense of another unit without tracking down whatever other problems there might be first. Any ideas anyone? Many thanks... kenny