Stick a paper clip in there, then touch your probe to the clip......
the reason why you go from 16(bat) to 4 with a test light, is to show that you have a good ground......terminal 4 goes directly to the ground that C340 also goes to.....if it didn't light, then you would know the ground is bad.....
You can make a ground anywhere, but you want to be higher up than the floor which is wet......find a dry spot drill a 1/8" hole......make a wire with an eyelet on the end......now just sand a little around the hole to bare metal where the eyelet will touch.....apply a little dielectric grease between the hole and eyelet(prevent corrosion), and then insert a self tapping machine screw, through the eyelet into the hole......attach the other end of the wire to your black wires, and solder them together.....