Car won't start.Installed new battery. Red Battery light shines and all electrical works, but the car won't start. Turn ignition, 1 second humming sound then nothing. Do you think it's the alternator or, hopefully, the voltage regulator?

Or neutral start switch?

Sixto
83 300SD
98 E320 wagon

In most cases the generator/alternator and its regulator do not crank the engine; the starter motor does.

As sixto suggests, the neutral safety switch can fail to close the starter energizing circuit. Another problem can be starting two threads, a day apart, on the same subject. One subject>one thread!