1981 Euro 500SEL no start 117-961 Engine

Wife took car to store and she came out and it would not crank ,nothing, no sound what so ever , got car home and jumped started it using a remote starter , so I knew it wasnt starter .
I then cleaned ground strap and installed new ground from battery to starter, still nothing. Battery, Alternator new and all working order.
Would my next course of action be replacing the ignition switch ? and also what role does the ignition module play, could this be the cause of the problem ?
I'll keep on looking in the forum for a solution, but in the mean time if anyone has any ideas please let me know