Once you sort out the lower board wiring by repairing any burnt or broken wires, you can de-solder the BLK/WH conductor at the jones plug that is located on the lower board. This may help to prove what direction the continuous point scoring is showing. The other thing to look at is the Zero Position Bonus Unit switch, which is located on the underside of the play field. Pic attached.

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OK, so the WH/BLK is carrying 50 volts from the 500 Bonus Advance Relay, (F17 on schematic), that runs to a N/O switch on the same relay to a BR/WH. That might be the conductor that you see melted. Gently start separating the bundles to try and trace that conductor.
That being said, the same WH/BLK also runs to the top rollover switches on the playfield, (located at the jones plug towards the back of the lower board on the left).
The WH/BLK also continues to the Alternating Relay to a M/B switch, (WH/BLK, Y/BR, Gray), and also runs to the Balls Per Game Adjustment located on the center of the lower board.

I would follow the burnt Br/Wh you have and see if it runs back to the 500 Bonus Advance Unit switch. I hate cutting bundles but you can isolate the damaged conductor and splice in new wire.

Check the Bonus Unit switch stack screws for tightness. This area proved to be a problem on mine and also found a broken soldered connection there. Just manually rotate the unit and watch the leaf switches open and close. Adjust if necessary.

That BR/WH looks like it runs from the N/O switch on the 500 Bonus Advance Unit back to the Outhole Relay N/O switch then to score motor 8B.

Left and right outer lane rollovers are controlled by the alternating relay. It is seen in the schematic at 24D. The circuit gets a bit confusing as you follow it up on the schematic. The 6 volts lamps for each are also controlled by the Alt Relay on the bottom left of that relay. Right outer lane lamp is green conductor, left outer lane is Brown-Yellow. However, for these lamps to work, the "Extra Ball" relay must be closed.

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