Headlights, since they require more power than the inside lights of a car.
Check headlights and dashboard lights first. If no/minimal power for the lights then replace battery.
A broken wire, a dead battery.
On most vehicles, the low beam headlights are the outside pair of lights. This is true when the headlights are side by side. When the two sets of lights are stacked, the low beam headlights are usually on the bottom.
Sounds like you need a new battery.
if you have no headlights the battery is dead or one of therminals is very corroded.
under the headlights On the inside of the markerlights
Then something is on pulling power from the battery. Check all lights and relays.
I would sue him/take the car back.
Because the power for your headlights is pulled from your battery. When the dashboard lights dim after turning on the headlights the dashboard lights aren't getting as much power as they were with the headlights off. With the motor running or off? The headlights draw a lot of current, but should not visibly affect other systems. There could be a problem with a worn out battery, an alternator that isn't charging correctly, or you have a bad earth between the battery and the chassis. Check the large cable connection between the battery (usually negative these days) and the bodywork, or between the engine block and the bodywork.
The battery stores just so much electricity. The lights use this electricity. Sooner or later the supply is depleted.
Headlights, interior lights, parking lights,Headlights, interior lights, parking lights,
Check the battery. Turn headlights on - they will come on with or without the key if it has power from the battery. If lights do not come on, the battery is dead (try jump-starting) or there is a bad connection (loose cable), or a short in the electrical system. If headlights come on, but dashboard lights do not light up - check the ignition (cylinder / switch)