Every SX4 seat I've ever owned was broken or about to break. Its a universal problem. The good news is that you have an 83, so the frame is much stronger than an 81 would be because of the recline mechanism.
If you look at the drivers side in comparison to the passenger you will likely see the seat a few inches back and leaning slightly to the left. The main rail has cracked. If the seat isn't back and over, but the cushion feels broken when you sit on it, like you're sinking too far, its just the zig zag supports that are broken. That's also a common problem but its easier to repair.
Taking the seat apart is done without removing the rails from underneath the car. I can do it in less than two minutes. Take the plastic cover off the side (two phillips screws), remove the three torx bolts you find there, then use a large screwdriver or pry bar to gently push the metal ring from the base over the center stud as you pop the upper half of the seat out. In the inboard side its just a metal post in a hole with a plastic bushing. It takes about a minute to get that far.
After you have the upper part off, then you will need to reach underneath the seat from the backside and pop plastic clips off the edge of the seat rail. On a seat that's already been taken apart (since some have metal wire clips from the factory) and with experienced hands the cushion pops off from the lower seat in less than a minute.
Now you'll be looking at your lower seat frame. Its held on to the rails bolted to the floor by four half inch drive nuts. It comes right off without touching the rails. You'll find the cushion supports look like zig-zagged metal wire. Most of those if not all of them will have their attaching tabs broken. That's an easy fix. Most seats will also have the main rail cracked. Push it back together, bridge the crack with a stiffener, and weld or bolt it back together. If yours isn't broken yet, just wait a few years. It will be eventually. Hopefully you can stiffen it before it breaks.
There are two different zigzag supports. Each seat has a mixture of both. I personally remove all the stretchy ones and grab extra stiff ones from other donor seats. I also stretch them one "zag" further and use a grinder to trim off the excess. Sometimes the hook is still intact and they just pop out of place or break a zag. For those I use screws to restrict them from popping out in the future. If the tab is broken I use screws with the heads overlapped over the "zag" wire to keep it in place.