I recently purchased a mint 1988 rx7 fc convert s4 and it has such low kms at 92,000 and its amazing under the hood but there are 3 main things wrong with it.

The fuel gague reads to about halfway and goes empty while I still have fuel in it.

The oil temp gague doesn't work at all and the car constantly thinks its being overheated

The Headrest speakers do not make any volume and I'm wondering if its worth fixing or if I should just invest in a subwoofer.

Any help would work, I'm more concerned about the overheating and fuel gauge than the speakers

 

Full service manuals including wiring diagrams are located here
http://foxed.ca/indexmobile.php?page=rx7manual

fuel level sender is probably broken. It has wrapped copper coils where the level sensor moves across, measuring the resistance which translates to fuel level.

Service manuals should have diagnostic procedures via multimeter for testing coolant temp gauge sender and oil pressure sender units.