This paper studies the strength, fatigue sensitivity, safety factor and lifetimes by means of structural and fatigue analyses of different models of rear doors upon the opening of doors and windows leading to severe fatigue fractures of the window motor components of rear doors. The simulation models were a standard model and other models. The other models, which are denoted here as models I and II, were modified versions of the standard model, with a rib of 3mm and a thickness of 2mm as compared to the standard model. The door was modelled with CATIA V5 and analyzed with the ANSYS program. The material of the rear door was cold rolled steel (DDQ). From the study results, the standard model and model I were confirmed to be less safe upon the opening of the door as compared to the opening of a window in terms of fatigue, but model II was found to be safe for both door and window openings.