I think the switch you identified in your photo is the Zero Position switch shown here on the left:
Bronco Subtract Bonus (resized).jpg
Note the yellow-red wire in the schematic and in your photo. If closing that switch causes your Bonus Unit Subtract solenoid to fire then there is a path from the yellow-red wire on the switch to the red-white wire (not labeled) on the very right side of the schematic. @pinballdaveh suggested checking the normally open J and AX relay switches as possible causes of the path. Ordinarily I'd agree if there wasn't a Score Motor 1A switch on the right of the schematic which isn't shown in your photo.
It seems unlikely that there is a short across the Score Motor 1A switch and the J or AX switches unless you are seeing other issues in the game. It seems more likely that there is a short through some other path not shown on the schematic. I'd start by tracking down the yellow-red wire to all five of its endpoints. If you're lucky you'll find it shorted through a bent solder lug or solder blob to some other wire at one of those five end points. If not you'll have to go through the same exercise for the wires (red-black, slate-red, etc.) on the other side of each of the closed switches (4B, 5B, J, etc.).
Alternatively you could try turning off the lights, doing whatever you did to create a spark on the Zero Position switch, and looking through the game to see if there is a spark anywhere else. That might lead you to the short.
/Mark