Hi Jaffa,
The On/Off switch for the lighting supply is the button on the throttle unit, and with the switch pressed in to the "On" position, full battery voltage (which can be up to 58.4V with a 48V battery) is supplied to the Red lighting feed wire on the control harness.
Unfortunately, full battery voltage is going to be much too high for that LED front light if it only has a maximum rating of 20V.
However, if the rear light unit is designed for use on a 24V truck, it should be capable of accepting up to ~29V (typical charging voltage of a 24V truck) in which case it should be OK to use it with a 24V battery, but not with a 36V or 48V battery.
Alan