Normally open switches on j and ax relays need to be checked for being closed.

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

Excellent observation. If you unplug the game and clip a meter to the yellow-red wire and the orange-black wire on either side of the Score Motor 2C switch what resistance do you measure? If the switch is open you should measure anything from 10 or 15 ohms to an open circuit. If you slowly turn the Score Motor until the 2C switch closes the resistance should drop to an ohm or less. Is that how yours behaves?

Your reading is fine. 0 ohms is the important measurement when the switch is closed, and something much higher when the switch is open.

But the measurement may be moot. When you mentioned 4B and 5B I looked right at the orange-black wire and the Motor 2C switch in the middle of the schematic above as the path where the short might be. I overlooked the top path with the blue-black wire where there are 4B and 5B switches in series. Since flipping either 4B or 5B fixes the problem that is the likely path where the short is, not the orange-black wire path. Sorry for the distraction.

Given your observations so far it may be that there is a short from the blue-black wire to the red-white wire on the right side.
Bronco Subtract Bonus short (resized).jpg
With the game unplugged measure the resistance between the blue-black wire and the red-white wire as shown by the arrow. Also measure the resistance between blue-black wire and the black-white wire on the other side of the switch on the J relay. An expected measurement would be 10 ohms or more. A short would be closer to 0 ohms.

If your measurement indicates a short, I'd repeat the earlier exercise and closely examine the blue-black wire. Find all five end points, see if there's any sign of a short (bent solder tab, etc.) between it and anything else.