What on earth is this and what is it for?

Looks like what is called a "power drop". Many vehicles have a similar coil of wire hidden up under the passenger's side of the dash for added on accessories such as police car lights, trailer lights, etc. If you're not going to use it you can just tuck it back under the dash. Hope that helps! Jim

Looks like an Ethernet port, does the vehicle have a type of computer onboard?

Just the ecu, and that’s nowhere near this, it’s under the driver seat. I thought Ethernet too, but it’s a ‘92…

I just remembered that some old amplifiers used a connection like that to connect the controls to the amplifier, some were powered some were resistance readers.

It’s actually 2 cables wrapped together. One is the Ethernet looking thing which leads to somewhere past the heater core toward the firewall, the other is this 3 fused wire plug that leads toward the gauges. (Photo) I unplugged the 3 wire connection and everything still works…

It is something that was added to the vehicle, not factory.

I’m aware. Otherwise it would be easy to diagnose.

Maybe a hard wired car phone, from the days before cellphones? Just a second thought. Jim