Sorry for not being as clear as i should have.
The smaller terminals on the solenoid are for trigging the solenoid on or off. One is for a ground wire, one is for an Ignition switch source, like a fuse which is live with the key to ON. It does not matter which of the smaller studs you use as + or negative. The exception to this is if there is only one smaller terminal, If this is the case then the solenoid gets its ground through the mounting bolts, and the one, single smaller terminal if for a swtiched/ignition 12v +
This trigger circuit, as an option, can also have another switch( illuminated) mounted on dashboard or somewhere near driver, placed inline on either the ground(-) or (+) wire, which then requires the driver to manually turn the solenoid on or off, Which is why I recommend an illuminated switch.
The fatter/ bigger studs are for passing alternator to house battery and the thicker cabling.
Keep in mind the solenoid does not NEED to be placed on the firewall in engine compartment, It can be placed anywhere where most convenient and for a shorter coverall circuit length, except on top of the flooded battery.
To pick a trigger circuit from the vehicle's fuse panel, One needs to find one of the fuses which is live when the key is ON, but not live when key is set to accessory. This requires a multimeter. Place multi meter on volts, and place the black wire on a grounded metal item, like the outside ring of a Ciggy plug, and then place the red probe on individual fuses until one finds an appropriate circuit.
Ideally this circuit would only be live with the engine actually running, rather than when actually cranking the motor or with key to ON. Some vehicles will have a such a Switched 12v commector floating somewhere under dashboard.
If one finds this stage to be a bit confusing or frustrating, then one can just take 12v+ from either battery and use the illuminated switch, and remember to turn it on and off, but this introduces the human error factor. But a glowing red light on the dashboard is kind of hard to ignore too.
The trigger circuit only requires the wire pass about 1 amp, as this is all the solenoid requires to fire the electromagnet.
If taking power for the triggert circuit from a fuse, make sure to tap the wire so that it is Already fused via the fuse block, and the wire is as thick as the rating of the fuse dictates. Like if one uses the fuse for the Hvac circuit, it would be a 25 or 30 amp circuit and one should then use 10AWG wire, which is very overkill for passing just one amp.
One could run thinner wire to trigger solenoid, but then that wire needs to have ian inline fuse to protect the thickenss of that thinner wire.
To make tapping a 12v ignition switched fuse in the vehicles original fuseblock.
Here is an ATC fuse tap:
https://www.amazon.com/s/ref=nb_sb_...O+fuse+tap&rh=n:2858778011,k:ATC/ATO+fuse+tap
Here is a glass fuse tap:
https://www.amazon.com/Wirthco-3020...qid=1487047987&sr=8-1&keywords=glass+fuse+tap
