The yellow wire is in common with the main lead (which I'm guessing is Battery +ve). This being the case, the yellow wire does not switch the solenoid and is superfulous to the switch. What the yellow wire does is provide a feed for an accessory that has since been removed. Seeing that it is a 6mm wire with a fuse holder I would guess either air compressor or fridge.

I'd be guessing dual batt. system for the switch, but that's without seeing where the rest of the wires go.