The main aim of this study is to understand the effect of thermal conditions on the fatigue resistance of one automotive Daytime Running Lamp (DRL) housing made of Polycarbonate (PC) material. Automotive lighting products are made of mostly thermoplastic materials. Thermoplastic materials have mechanical properties varying significantly by temperature. As a result, thermal conditions at service life must be considered before evaluating the mechanical performance of automotive lighting products. In this study, thermal finite element analysis (FEA) has been done in order to understand the temperature distribution on DRL components at different thermal environments. Thermal map files representing the temperature distributions of the DRL components have been extracted and entered as load inputs into the modal FEA to find out the resonance frequencies. Using material properties varying by temperature, resonance frequencies of the DRL have been found by modal analysis and compared. Also the effect of light function on thermal condition, and consequently on mode shapes, has been compared by modal assurance criteria (MAC). Furthermore, a random vibration fatigue analysis in the vertical direction has been performed by an input power spectrum density (PSD) G acceleration load for all cases using temperature-dependent S-N fatigue curves of the PC material. According to comparison results, it is understood that there has been a significant decrease in the fatigue resistance of the PC housing because of mechanical material property degradation by thermal effects. Taking into consideration the well-known condition, higher acceleration loads at low frequencies during service life that trigger the resonance frequencies of the DRL housing can be hazardous and may result in partial or complete failure of the DRL. Hence, structural improvements must be considered and applied by considering extreme thermal conditions during service life.