I don't have S class schematics, but on E class of same year, the k38 is behind the speedo cluster.
And the NSS can be jumpered under the dash, by the brake pedal.

The starter series circuit goes like this

Bat power to Ignition sw> to NSS ,> to K38 relay contact, >to plug I mentioned under hood, to starter solinoid.
I use the plug under hood and bring 12v from battery to the center terminal.. if that engages starter [ as it should ] , I then know the problem is upstream from that test point. meaning NSS/K38/K38 fuse/igfn sw.
if no starter activation at the test point , it is down-stream [ meaning starter motor/sol/wires ]
look around for a 3 wire plug near the brake res. The cenetr wire is V/Yl on the E class.
..or maybe someone here has an S classs/140 schematic..that is the best bet .