A d-c to d-c converter circuit (DDCC) (10) that is comprised of a passive thin-film hybrid circuit that includes an input circuit (12), a voltage regulator circuit (14), a comparator circuit (16), an oscillator circuit (18) and an output circuit (20). The input circuit (12) is applied an input d-c voltage (11) from the vehicle's battery circuit (50). The input voltage (11) is converted by the DDCC (10) into an output d-c voltage (23) that is suitable to operate a set of LED tail lamps and/or a set of LED turn-signal lamps. The DDCC (10) is designed to replace a prior art heat-producing, voltage dropping resistive element (Rx) that drops the battery voltage to a suitable level to operate the LEDs. Additionally, the DDCC (10) is easily spliced into the vehicle's electrical wiring cable (54), requires a small mounting space and produces no significant heat.