First and foremost I can't afford to see a mechanic rn. The mechanic I spoke to said it'd be 80$ just for him to identify the problem. Which is probably fair but is way more than I can afford atm.

My car is a 2002 Hyundai Elantra. A little over a week ago I went to start my car and the practically brand new battery was dead. I had no idea why, I checked the headlights and they were off. I had to run to work so I charged the battery and left. I ended up needing a jump after work bc it was dead again. I was worried it was a battery problem but later that night I was grilling for dinner and in the dark I could see the red light of the tail lights lighting up trees behind it so I went to see what was wrong. All I could tell was that even when the car was off and the key was out the brake lights / tail lights stayed on at all times.

I asked my dad and he said he didn't have any idea other than that the best way to preserve my battery was to disconnect it so it wouldn't be drained. I did but that doesn't solve the problem and it's really inconvenient and I can't imagine it's good for the car or battery. I've tried googling but haven't gotten much info. Does anyone have an idea or what might be wrong and how I could fix it?