1991 t5 nutural safety harness problem
 

Don't quite understand. Is the plug you're holding on the wiring harness? If you don't mind being able to start in neutral (there's probably a clutch pedal switch anyway, maybe), jumper the two wires which the nss in the transmission normally closes in neutral.

It sounds as though the harness may have been changed when the C4 was put in. Now, according to my 1990 Shop Manual, the only electrical connection to the T-5 transmission is a back-up lamp switch. The neutral start function is handled by a switch mounted on the clutch pedal. You need to determine if that switch is still operational, trace it's wiring, and connect the switched wire (not the one from the ignition switch) to the starter solenoid under the hood.

I just took a picture of the 2 harnesses that pass from the cabin,through the rubber grommet,to the trans tunnel and
they appear shaped differently than the ones shown in your picture.I was lying on the driver side looking up at the two harnesses.My car is a 91 t5 car.Ive had the engine out of this car for awhile so the harnesses arent plugged into anything for me to reference.One has a 2 pin connector & the other one has a 4 pin connector coming out at that location.I know its kinda hard to see in the disorienting picture,but if you look directly above my middle finger,you can see the same rubber grommet thats shown in your picture.One harness plug is round & the other is round with one squared off portion.Do either one of my harness plugs look like they will plug into your ngs (neutral gear switch ) Is your car an original t5 car or could it have possibly been equipped with an aod instead & be the reason why one of your plugs appears different?? If you're not sure,check the trans code on the white certification label.Its located on the end of the driver side door.
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https://cimg4.ibsrv.net/gimg/mustang...69a896744e.jpg

Both of my harness plugs,going by their shape,appear to match the harness plugs that are coming from the actual ngs and reverse light switch.The pictures below show the ngs and rls and the harness/plug attached to it.If you look at their plug shape and pin count and compare them to the harnesses shown in my previous post,youll see they appear to match.

https://cimg1.ibsrv.net/gimg/mustang...1823d43e50.jpg

https://cimg7.ibsrv.net/gimg/mustang...a5089ac2cd.jpg

The plug you're holding onto in your very first picture looks more squared off to me vs being more round like mine &,unless Im not seeing your picture clear enough,your plug also appears to have female pins in it vs male pins,which are required to mate up with the female pins in the ngs attached harness plug thats shown in my last post you quoted.IOW- the plug shown in your first picture looks to have the wrong shape & wrong pin type (female instead of the correct male pins) to connect to the ngs harness plug (which has female pins) thats shown in your second picture?? Are the pins inside the plug in your first picture female or male?? ​​​​​
Are the gray harness plugs that are connected in your first picture for your reverse lights?? Im thinking yes.....

The following might make things a little more clear.Take a look at the pictures below & pay attention to each plugs shape and the location of its locking tabs.

The ngs plug has 2 square & 2 round sides + female pins.
Your plug has 3 square & 1 round side + female pins.(they look female in the picture anyway)
My plug has 2 square & 2 round sides + male pins.

Also look at the plug locking tabs.1 is on a square side of the NGS plug & 1 is on a round side of the NGS plug.

So if the NGS plug requires a 2 square/2 round sided plug with locking tabs on 1 square & 1 round side of the plug,which plug will connect to it,out of the plugs listed above & shown below?? Your plug or
my plug???


(Your Plug)
https://cimg8.ibsrv.net/gimg/mustang...07d37e02b7.jpg

(NGS Plug)
https://cimg9.ibsrv.net/gimg/mustang...ba7c745b8f.jpg
​(My Plug)
https://cimg0.ibsrv.net/gimg/mustang...ae52074588.jpg