As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MAIN CABIN LIGHTING IS LOST WHEN GROUND POWER CABLE DISCONNECTED AND A-CON APU POWER. BITE CHECKED BPCU SYSTEMS, CHECKED OK. FOUND THAT OVERHEAD EXTERNAL POWER SWITCH HAD TO CYCLE SEVERAL TIMES IN ORDER FOR GROUND POWER TO COME ONLINE. REPLACED GROUND POWER SWITCH S12.