Since my ‘75 K25 probably matches your ‘75 K20, I think I can help.
- the clear fuse holder on the left is the fuse (25 amp?) for the HVAC blower high speed. If it blows you will have no high speed on the blower. On mine the fuse holder melted and I had to splice in a new one.
- the black fuse holder pointing down to the left is a 4 amp fuse for the ammeter in the dash. The ammeter uses the wiring loom as an ammeter shunt, so there is one fuse on this side and one near the alternator. The ammeter measures the voltage at each fuse, and uses the voltage drop to estimate amperage flow.
- one of those other two wires off the junction point is a fusible link. According to my wiring diagram it should be in the charge wire from the alternator. It is an old style fusible wire where the fusible part is internal. If it starts to blow you will see the outside of the wire start to blister and bubble, and then finally burn. I made a mistake and hooked up my battery terminals backwards once, and it burned that fusible link.
The other wire on the junction is power from the starter lug, and it is a 10 gauge wire with a fusible link in the starter end.
You can buy fusible link wire at NAPA, but a better option is a pick and pull with a bunch of late era squarebody trucks and GMT400 trucks. GM started using links with a plastic barrel that is clearly marked (usually in millimeters), so the links are easy to see.
