Your oil pressure gauge is actually just a go/no-go indicator. A pressure switch on the engine closes above ~7 PSI and grounds the gauge signal circuit thru a current-limiting resistor so that the gauge needle ends up positioned somewhere in the NORMAL band. When your oil pressure drops below the cutout pressure of the switch, the signal circuit opens and the gauge dives into the red. You can bypass the resistor and replace the switch with an actual pressure sensor to make it behave like a real gauge. Search for Ford oil gauge mod.

The electric rear window is a Mark 3 thing, not factory. Check for power at the motor with the switch pressed. If none, then work your way back til you find the problem.

Jumping speedo could be a few things... dirty VSS, failing PSOM, or failed RABS controller.

Doubt it's an air horn, why would anyone put the controls on the side of the seat? That is the factory location for the lumbar support control. Push the combined rocker switch/air valve in one direction and an electric pump under the seat inflates the cushion in the back part of the seat behind your lower back. Push it in the opposite direction and the valve portion of it bleeds air out of the cushion. If you can hear the motor running but the cushion won't inflate then it's usually because the air bladder inside the seat back has cracked/split or the hoses leading to it from the pump are dry rotted.

 

I recently did my window motor and regulator. I don't know how anyone would even get bolts onto the nuts with everything in the way, but there are many more skilled than I. I used 1/4" steel pop rivets with a BIG hand riveter.
These window motors go bad often, but instead of buying a new one there's an easy fix. With the motor out, remove the housing screws from the circular part and you'll find the broken bits of the plastic gear that drives the arm. Clean these out and place ball bearings or even 1/4" nuts in the each of the three sections. This'll give you solid purchase from the rotation of the worm gear. There's several you tube vids on this but once you get the broken gear out you'll see exactly how it'll work.

 

Oh, and the sliding rear glass is a Mark 3 addition. If it's like mine you'll follow the circuit all the way up to the switch and end up taking down the headliner. Before you do all that, check the fuse. It's an inline fuse and there's no way to identify without trial and error. Mine is found, exposed, zip-tied in the wiring under the dash near the fuse box. It's a fairly heavy gauge light blue wire and the inline fuse holder should be visible without removing any trim panels.