Welcome! My oldest 240 is a '79, but I've seen some 77's in the yards and up close, so I think I can relate to most of the changes in that time frame.

1. The behavior of the oil light does sound like a sender or the black wire is shorted, except for the flickering at high rpm (scary) and the history of the wiring harness rot that didn't really start until '81. Were it mine, my first step would be to get a real gauge on it soon. Any pressure gauge you can adapt to the 1/8" NPT connection where the sender is screwed in will do. If indeed the pressure is low, you'll be dropping the oil pan.

2. I don't believe the tach was a common problem, but it is old. It connects to the coil, and my wiring diagram shows the color as black for 1976. The only other wires are power and ground - short jumpers to the instrument cluster.

3. Brake lamps both out: In order of likelihood - fuse 8 is dirty, bulb out sensor is damaged, brake light switch has burned contacts. Nothing beats a test light, though, to find the culprit. Guessing just wastes time.

4. Sure the brushes could be short on the blower, but check for power at the switch before you go having fun pulling the dash apart. It is a yellow wire from fuse panel to switch. There's no resistor unless someone has already changed the blower motor.

5. The odometer is a common trouble, where the gear is cracked and slips on the shaft, or like in my experience, the gear is not cracked but the shaft is worn thin where the gear rides. The cracked gear is often fixed by glue, on this mechanical speedometer, and in my case, with the worn shaft, I just reversed the shaft to move the gear off of the worn spot.





6. If you remove the sheet metal cover from the wiper motor gear box, you will see the contacts that park the wipers. That is the easiest place to look first, although the parking circuit still requires contacts in the column switch. The most commonly reported trouble with wipers is the failure of the ground circuit, through the mounting bolts at the firewall, but I think that would show up in normal operation.

7. I have no experience with the different latches used in 240s prior to 1979, but the manual shows an adjustment for the internal release.



Most of this info is also available on line from k-jet.org in case you haven't seen that resource.

Best of luck to you.
--
Art Benstein near Baltimore

The journey of a thousand miles begins with a broken fan belt and leaky tire.