the fully automated climate control unit is a known weak point on any mercedes benz sedan, Them germans have a mental block in this area I think.
You can either get one from Ebay and have it shipped to Pakistan or if its not that much knackered and only requires resoldering, get it resoldered. The other most happening problem with the CCU is the air sampling motor failing which causes the CCU not to regulate the temperature correctly it is good for about 10 years of service. The brushes wear out causing it to stop, replace or repair as you deem workable.
The heater control valve and pump (if equipped) is also a problematic area if the car has been run on bad coolant (green silicate stuff) or plain water. The diaphram tears apart and the system cannot work, because by default the valve is open (heater mode), it closes when cool or cold is called for. Bypassing it does not work and can create giant air locks in the unit too. There is a kit available for repair - its about USD 50/-
For diagnosis, find a freindly workshop that has a mercedes multiplexer reader (most good shops use the X-431 and some models also have provisions for the 38 pin connector too) - get it diagnosed and repair accordingly. Shnwz motors have sworn an oath to rip your butt for money, a good independant shop would be in favour if you are good with making sense on auto workings. Do not let any average joe use his worn out tools on your car unless you are planning to trash the part being repaired. Make an inventory of specialty fluids, pastes and greases to take along when you get your car worked on by your selected benz mechanic. That ensures that the repair procedure was correct. (e.g. when servicing brakes and front wheel bearings) btw the front bearings are correctly set by a dial gauge unless the mechanic has installed about a 1000 bearings on mercedes and has developed the feel for it. They cannot be set by andaaza like regular cars because of their proprietary gland nut.
Its not that difficult to maintain a good benz - but its a nightmare if you bought a basketcase, unless you paid dirt money for it.