1982 123 280E Manual central locking problems

Hi all
I am having problems fixing the central locking on my 1982 280E 123 right-hand drive. All seems well inside the doors, and have checked for vacuum leaks as far a possible, but to no avail. The speed control unit is
also not working, is it possible that vacuum is lost inside the speedometer consol or if anyone have suggestions, it would be great. BTW my car has over 250 000 km, about 154000 miles and still going strong. I get fuel consumption of 12.5 km per litre in city driving and 10.5 km per litre in highway driving, which is excellent considering the car is over 20 years old.

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