What is NSS?
 

Seen several threads describing the NSS is a lock-out - and a car (E420) would not start.

What is the NSS, and where is it located?

The NSS and OVP (Over Voltage Protection) are things I think I should understand. The OVP needed changed in the 300E - battery kept loosing a charge.

NSS= Neutral Saftey Switch. If it is bad you can only start the car in Neutral, not Park.

Only start in N is just a test for both linkage bushing wear, NSS alignment, and a possible NSS contact failure...but you can have a perfectly good NSS with a start in N only condition.

On the OVP , you do not have one ,as you have a V8 engine chassis .
Not sure on the exact chassis as you list it as a 95/E420 on another thread , and you have it here as a 96/E420. A 95 would be a 124 chassis, but a 96 would be a 210 chassis and they did not have an E420 in that chassis until '97...
Which chassis is it?

It's located on the side of the transmission. Follow your gear shift lever to the shifting rod to the NSS - looks like this - http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2EV00YCOB2FB1B802X&yearid=1994%40%401994&ma keid=MB%40%40MB%40%40X&modelid=E%2D420%2D001%40%40E+420+Sedan&catid=J%40%40Transmission&mode=PA&subc atid=J4050@@Neutral+Safety+Switch&source=www.peachparts.com&clientid=catalog.mercedesshop

My typo - 1995 E420
 

The last of the w124 years (I think)

1995 E420 - with all the horse power

I changed the OVP on my 1990 300E

That would be the last year of 124 chassis..I think the confusion comes b/c your Sig says 1996..
Either way, no OVP on that chassis.