My car wouldn't start earlier this winter and I tried to test the NSS. Everything I read (in MB manuals and online) said that if the car started in Neutral but not Park, the problem was the NSS. Mine wouldn't start in either so I was convinced it was something else.
Mechanic replaced the NSS and it has been fine since. He said the switch can fail and the car will start in Neutral -OR- not start at all.
I haven't seen the actual switch, but if it is a two-wire switch you should be OK jumping the two terminals to see if the engine turns over. I believe in my '86 300SDL (W126) the switch was on the left side of the transmission, under the car.
Depending on your climate, this could very well be the problem. Mine was original and subjected to salty water and cold winters for 15+ years. My guess is that one day the corrosion finally got to it and killed it.
'83 300SD 335,000km (207k) mi
'87 560SL 163,000km (101k mi)
'86 300SDL 356,000km (220k mi)
'92 500SEL 250,000km (155k mi)
'90 300SL 140,000km (87k mi)
'01 S430 260,000km (161k mi)
'03 SL500 167,000km (104k mi)
'07 GL320 CDI 4MATIC 289,000km (162k mi)
'07 S550 4MATIC 170,000km (105k mi)