I’m having progressively worsening issues trying to start my s350 - initially when trying to start I noticed that if glow plug light illuminated at ignition position 2 it would start no problem - if it didn’t I used other key and it generally would eventually illuminate glow plug light and start - I fitted a new Bosch agm battery just in case it was the old battery tiring - no real effect from battery change - then I found if I run a tyre compressor from cigarette lighter it would then trigger glow plug light and start - weird but true! - now the only way to get glow plug to illuminate is to put battery charger onto car - battery at 12.54 volts prior to adding charger - with charger on and charging at approx 6A turn ignition on and glow plug comes on and vehicle will start - don’t know if this is a bad earth somewhere or starter motor relay / or starter itself - no clicks noises or anything when trying to start when glow plug does not illuminate - car has done 55k and is in good order - normally!!! - any help or ideas would be much appreciated!