If it's lacking, trace the wire checking for voltage at various points with a test light needle probe by piercing the insulation. Maybe the taillights and brake lights work but turn signals don't. Try replacing the flasher unit. If just the brake lights are out, replace the brake light switch. On most model cars, it's a mechanical switch near the brake pedal.
Earlier models have a hydraulic switch at the end of the master cylinder.
If one headlight is inoperative, replace the bulb or the sealed beam unit, clean the connector and tighten. If lights are dim, check the connection between the headlights mount and body ground. When both headlights are out or flash on and off, check for a loose connection at the dimmer switch and the headlight switch. Check for voltage at the dimmer switch with a test lamp. Should the test lamp fail to light on the hot side of the dimmer switch, check wiring from the head- light switch to dimmer with a test lamp.