Yes
The short answer is yes. Both the 1989 and 1990 models use the RS5F50A manual or RE4F02A automatic transmission.
inside the transmission
YES
It'll fit, but it probably won't work very well, as a 1996 transmission would be wholly electronic, and the 1990 transmission would not.
It doesn't have one, it has a switch on the clutch pedal. It also has an interlock switch on the dash, turn the ignition switch on and press the interlock switch and it will bypass the clutch switch.
no
Yes, even though you do not specify the front or rear bumper, both will fit. All d21 Nissan pickup trucks are referred to as "hardbody". They were manufactured, in the U.S., between 1986 and 1997. Most parts are interchangeable between the years, the only real difference is the bed length between the standard cab and extra cab, they are not interchangeable.
Yes, engines are different but the tranny's the same.
On the underside, passenger side, of the transmission.
it is located on the transmission itself
No because the driveshaft and transmission are diffent sizes.
Yes