I have a 1994 Bridgestone with 7x3 Shimano STX-SE components. My rear shifter stopped returning to the original position. I took it apart and found a broken piece inside. I’m pretty sure that broken piece is what is causing the problem. I found a shimano shifter on Amazon that looks like it would work. My questions are these:
1. Are all Shimano 7 speed shifters compatible with my bike? I don’t want to change anything but the shifter.
2. Should I replace both the front and rear shifters? Or just the rear? It’s only about $8 price difference between the rear only and both front and rear.
Here is my original problem:
https://www.youtube.com/watch?v=pkq-l1ZhseE
Here is what I plan to buy:
https://www.amazon.com/gp/product/B00...=ATVPDKIKX0DER
Any thoughts or advice? I’ve never done this repair before.

I see from your video that your bike has cantilever brakes. The shifters you link to are integrated Shift and brake levers. The problem is that the brake levers are v-brake compatible and will not work well with your cantilever brakes. So you should be looking for 7 speed shift levers alone. Any Shimano 7 speed shifter will work properly with your rear derailleur setup

Comprehensive video!

most at shimano shifters of that era that are for 7 speed will work, the throw (1.xxmm/shift) is same. As for buying both, if you like, but I find that my shifter for the front mech don't do any where near the work of the rear so stay in good condition for much longer.

I use these on two of my bikes and they have worked well for me: https://www.amazon.com/Shimano-Acera-...himano+7+speed It comes with cable and housing as a bonus.

+10 Wrong shifter choice, V brake model and integrated with brake levers to boot.

Personally, this is all I have used on older MTBs, both keepers and bikes that moved on to new homes. Reliable, simple, friction front (my preference), cheap.

https://www.amazon.com/Shimano-Shifte...642631&sr=1-13

Okay I'm about to order these instead:

https://www.amazon.com/Shimano-ST-EF6...pr_product_top

Although it says v-brake compatible, the descriptions explain there is a setting for cantilever brakes. The price is very good...

That should work.