Welcome to HEF reverrock. :drinkup
Ground controls: wht/yel 48-1 leaves start switch and goes to terminal #1 in lower control box, wht/yel 48-1 leaves terminal #1 and goes to starter relay. You've confirmed this circuit works.
Platform controls: With ignition on (E stop pulled out, foot switch not depressed), make sure you have voltage on yel/red 2-9 going to start switch. If no power, you have problem with either foot switch or cables. If power present at start switch, wht/yel 48-1 leaves start switch and goes to terminal #1 at platform box. Black wire from boom cable connects to terminal 1, carries start power down boom and connects to terminal #1 in lower control box, the circuit you've confirmed works.
If you have power from upper start switch sending power to terminal 1, but no power getting to terminal 1 at lower controls, you have a boom cable problem.