You have 4 wires TO the step
1. RED large wire is 12 volt positive
2. YELLOW wire is 12 volt when Ignition key ON
3. WHITE wire will have 12 volts when the switch by entry is OFF or ON. Check both modes
4. The BROWN is the ground signal from the Magnet.
IF you have 12 volts at the RED and WHITE(either way the switch is turned) and the step goes in and out possible control module. IF the YELLOW wire has power with Ignition OFF, then the step will operated as it is right now. Disconnect the yellow and the step will probably function as designed. Then trace the Yellow and connect it to Ignition ON only. Doug