So looking at that photo, I see a thick cable attached to the starter motor switch. That cable should go to the negative terminal on the battery. If that’s the case then your system has been wired based upon an “interrupted ground” principle. That is, as NK has stated, the complete path from battery negative terminal through all the car, through the frame and back to the positive terminal on the battery gets interrupted when the switch is open.
That open cable from the switch needs to ground to the frame somewhere, in a large contact surface free of paint and fixed with a tight bolt. In that area of the car the only bolt I’d think would be appropriate would be one of the bolts holding the steering box to the frame.
But - with those Float-a-Motors you need a thick cable jumping from the engine to the frame, because the engine needs to be grounded and the FAM forms an insulator (no metal-to-metal contact, the rubber pucks insulate). If you have a separate jumper, then that cable from the switch could be installed on one of the starter motor mounting bolts. Circuit would then pass from the frame and engine, through the switch, then back to the positive terminal of the battery.
If you DON’T find a jumper from the engine to the frame then whoever installed the FAM is likely assuming (or perhaps never thought about it) that the engine conducts to the frame through the front engine mount. That’s a very tenuous electrical path, I would not trust it. You need to install a jumper.
As an aside: check the battery positive terminal to make sure there isn’t a second cable or strap from the terminal to the frame (stock installation goes from the battery to the center frame crossmember). If there is then your switch will be useless. You need to run the ONLY connection from the battery to the frame through the switch.
