Welcome.
0) Always search before posting. As you noted most topics have been covered in the past.
1) See if the headlights can go up and down manually. See your owners manual or look near the lights to see how. If they are mechanically broken, that will lead to a different set of steps than if there are electrical issues.
2) You have a short. Google "how to find a short" and you'll find a lot of tutorials.
3) See if the speedo is plugged into the back of the cluster. Again search as removing the cluster is discussed. Someone may have disconnected it to falsify the mileage. The cables do break and you'll learns about people's experiences in other threads.
4) Mazda didnt discover how to solder until the late 90s! It's a common problem and probably not worth the effort. RainX for the win!
5) Use a volt meter and see if there is power getting to the radio. If there is, then the radio is dead.
The Factory Service Manual (FSM) is available as PDFs on the internet. You'll find them and much more in the
Archives.
Good luck with your project