Front seat electronics are a common weak spot: the pneumatic multi-contour seat bladders (and their vacuum lines/pumps) fail, seat heaters go inoperative, and power-seat motors become finicky. Buyers specifically ask about seat heaters and bladders because they are known trouble areas on the C55/CLK55 generation.
Seat bladders, heaters and multi-contour seat faults
Mercedes-Benz CLK55/CLK63 AMG (C209) · ownership