Grounding mods are a good idea.

I don't know what you mean by "both locations". Spring Break launches all balls from the shooter lane.

Does it fire balls from the outhole into the trough correctly?

You can try grounding a tab on the relevant transistor on the driver board. If this makes the shooter lane kicker fire, then there's a path from the driver board to the non-banded side of the diode on the shooter lane coil. If this doesn't work, there is a break in the wiring somewhere, and you might be able to find it with continuity checking and a multimeter.

You can also try grounding the non-banded side of the diode on the coil. If this works, you've got power, although it sounds like you've already tried that.

Having a manual, and knowing which transistors have step-up transistors in your game, is a big help!