Hey man so I’m out on vacation for the time being but we narrowed it down to one of two things. A fault In the wiring harness (the most likely cause). This was found by checking the 12 pin plug on the black/ blue wire and the red wire and seeing a low/ no power output and determining that there was a problem with the wire. The second less likely cause is the circuit on the motor RSE Is sending me both as we speak.I just received my step sliders last week and installed today. I have same problem with driver side not activating on initial check. Harness ran, zip tied, and connected. Calling RSE tomorrow. Extremeterrain.com's answer is to send it all back. I really don't feel like uninstalling this entire setup. Their customer service was zero help. I won't be ordering from them again. Will rely on RSE tomorrow. From everything I've heard, they are top notch.
Will install sensors and magnets after speaking with them. I do t want to get to far ahead of myself. Eager to hear OP's resolution also.