Solar regulator with 2 wires for vehicle battery

[fred flintstone]

Just removing my old solar regulator and replacing it with a Victron dual charge controller.
I took all the wires off my old controller and have left the starter charge wires in situ for this photo.
1 wire obviously goes to the starter battery but the other wire disappears through the floor.
Anybody know where it disappears to or what it is for?
Also, the wires seem very thin. I have always used 4mm in the past.
Any ideas?

 

[newtonz]

Has anyone suggested that the wire through the floor goes to chassis connection, to provide path back to can battery?

 

[fred flintstone]

Has anyone suggested that the wire through the floor goes to chassis connection, to provide path back to can battery?

Traced the wire through the floor, it went outside along the chassis and connected to the starter battery.