What was found
CREW REPORTED PASSENGER DOOR WARNING AND SERVICE DOOR CAUTION MESSAGES AT 800 FEET AGL AFTER TAKEOFF. CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND FOUND CARGO DOOR WIRING HARNESS BROKEN (CARGO DOOR INDICATION HAD PREVIOUSLY BEEN DEFERRED PER MEL). MAINTENANCE REMOVED CARGO DOOR WIRING HARNESS AND PERFORMED CHECKS. AIRCRAFT RELEASED FOR SERVICE. MAINTENANCE SUBSEQUENTLY REPLACED CARGO DOOR HARNESS AND CLEARED CARGO DOOR INDICATION DEFERRAL. ALL WORK ACCOMPLISHED BY MAINTENANCE USING AIRCRAFT MAINTENANCE MANUALS.