the 84-88 Extra cab really isn't intended to have a back seat IMO. I had my 84 X cab for 5 years. I did install a jump seat out of some random truck I found in the boneyard (don't remember what since I sold that truck 21 years ago) so I could put a car seat back there.
about all you can really do is add some speakers or storage back there.

 

Yeah it it is slightly better than the 90's era supra, which puts your knees under your chin due to seat to floorboard clearance, it has the metal structure which is good for storage and pumped up audio. You can't really do much in the way of side facing seating or more leg room unless you limit the front seats and hack out the structure

Bottom line its for kids or storage, unless you are very short and willing to hack out the boxes in metal.

 

What type of "improvements" are you looking for? A tool box and rear speakers seem like fantastic improvements to me for the back of an xcab truck.

3rd gen pickup had jumper seats and seatbelts, the 2nd gen only had a storage shelf.

I used to ride in the back of my dad's old 2nd gen all the time growing up, but nowadays I'm sure you'd get ticketed for no seatbelts in the back. In my 3rd gen xcab only once I had 2 adults ride in the back - never again in a 5spd truck that I normally drive with the seat all the way back, lol...maybe for my kids if the current laws didn't require car seats until they're basically teenagers, lol.

I remember my uncle's Nissan Hardbody had jumper seats that mount off the side of the cab instead of the rear like on the 3rd gen pickup -- perhaps something like that could be installed. You'd likely have to add some structural rigidity where they mount as the cab wasn't designed for this...and seatbelts...

 

After a week the level of interesting things is as high as the seat backs .

I find all kinds of things when I am hunting for something.