Just from looking at the schematics in the manual, the start (credit) button connects to the switch matrix, in theory you can tap off of any of the switches for the row and column. The green/brown is definitely tied to the plumb bob. The white/orange runs in the wire harness of the cabinet to the back of the game and then up to the playfield. I think the C-jump upper ring switch may be the closest.
For MPU IJ10-3, that is row 6 in the matrix, so run to any of the closest playfield switches in row 6 (white/blue wire) - probably top lane number "8" switch.
I can look at the rest later on my game, so if you still have questions shoot me a message later today.