First lets confirm your current wiring setup. I'm assuming terminal 1 in the structure is connected to your power source, terminal 2 is connected to the isolated rail, and terminal 3 connected to common ground.
In this scheme, you can insert a SPST (simple on off switch) between terminal 3 and its grounding source to independently turn the light on and off.
If you truly want to use a separate distinct power source for the light vs the actuating coil, then you are permanently modifying your structure to isolate the lamp from the coil.
Note: If you already have a illumination bus/circuit on a switch, you probably can't tap into it as is because it is likely by convention switching the hot side while terminal 3 is the grounded side of the circuit.
