seat control stopped working, checked fuses and loose cables,all seems tobe ok on 2000 Mercedes-Benz C280

up untill 3weeks seats were working fine, then all of sudden it stopped

I would check for power and ground at the switch and then power and ground at the seat motors when operating the switch.
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I had researched that the electric seat relay (in the trunk area) has cold solder joints and should be replaced. If you are handy with a soldering iron, you could try and repair it yourself. The part is around $30 US.