You are are at the mercy of my fading memory as I did this 7 yrs ago and have since sold that Cruiser so I can’t a look. Buy the relay @ Napa or similar, don’t get a cheap one. The black cover is just that, not necessary but a good idea for protection & to be tidy.
If memory serves this is how I wired mine. Use whatever color you want, but do use the gauge suggested. My picture shows 4 different colors of wires. Yellow, red, green & black. A 4 pin relay is numbered 30, 85, 86, 87.
Unplug your wire from the starter solenoid. That’s your trigger, it gets power when you turn the key to start. 14g is fine this wire will connect to 86. Then attach a new 14g wire to that same attachment point you unplugged & connect to 87. Then run a new 12g wire to positive side of you battery & connect to 30. Then a new 14g wire to ground & connect to 85.
In my picture it is as follows.
30. Yellow 12g WITH A IN LINE FUSE to positive side of battery. USE THE IN LINE FUSE.
85. Black 14g to a good ground
86. Red 14g connect to the WIRE you unplugged (trigger)
87 Green 14g connect back to the ACTUAL solenoid
So there is always power to the relay via yellow wire and why you MUST have an in line fuse. This power will go nowhere until you trigger it with the key via 86 and then it opens up 87 and gives FULL power DIRECTLY from the battery to the starter. Varoom!
Here is a pic I picked up online. It’s describing Lights…in your case it will be your Starter.
There you go. It sounds intimidating for a novice but it’s actually very simple. Good luck.
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Units like this are very common too as many older American cars used these.
The new heavier battery cable connects to the larger side posts (one to the battery, the other to the trigger terminal on the starter solenoid. One of the front posts is where you connect the OEM trigger wire that used to go to the starter.