OK. I can touch on some things with the additional info.
The wiper motors always had their wire harness, and yes some years/options the connector plug at the bulkhead has wires for other circuits. We can see what circuits go in that connector by looking at the factroy wiring diagrams. These are in the back of the electrical section of the 1974 Chassis Manual and the Body Manual.
It may be possible to route the tach circuits the same as a the factory did. We can come back to that. There were lots of variations by factory.
Seatbelt interlock will prevent the starter relay from working. The starter won't crank. Everything else will still work.
This will make more sense when looking at a schematic for the power supplies.
The 'alternator' gage is an ammeter showing current flowing in or out of the battery.
If the battery is discharging when the engine is running, then the alternator is not working.
I suspect they named it 'alternator gage' back in 1960 to emphasize their cars and trucks had alternators not generators. Ammeters were pretty standard back then so everyone was familiar with what they indicate. The normal position of the ammeter varies depending on what the battery is doing but most othe time its centered - zero.
If the car had a leak in that area, then the panel and connections get messed up. I'm not sure how common that problem was in '74 but the early 70s cars had a reputation for that. The rest of the factory gages are completely seperate. They get power through the key switch which is then limited to an average 5-6 Volts.
Here's the basic scheme of a '74 Duster with standard wiring. There were some heavy duty options, most commonly on cars equiped with electric rear window defrost grids. The circuit concept is the same, but the main power (from alternator and battery) goes through grommets in the firewall. If you're car has that you'll have to adapt the standard harness as the factory did.
The battery and the alternator output are joined to the other circuits at a welded splice hidden in the harness. That's the solid black circle. Since the battery is connected to all those wires, they are hot at all times. Dome light, brake lights, headlights, horn, all work even with the key off.
When the key is in Run position, the power feed to the key switch is connected to both the Ignition run (J2) circuit and the Accessory circuit (Q2). Wipers are on Q2 but have no fuse. There's a circuit breaker usually in the switch.
The ammeter is a solid metal plate with two studs pressed in. When electrons are moving through the plate, the needle is deflected by the small electromagnetic field generated. No current, no deflection.
If the ammeter studs are loose, the ammeter must be replaced or repaired. Electrons can't jump gaps or go through corrosion without generating heat and loosing power. If the ammeter shows near max either charging or discharging, something is very wrong. Shut down and if that doesn't solve it, disconnnect the battery.
