If you have a parasitic car drain, the first thing you need to do is identify where the drain is coming from. This can be done by disconnecting the negative battery cable and then using a multimeter to test for current draw. If there is a current draw, that means there is something in your car that is draining power from the battery even when the car is turned off.
Once you know where the parasitic drain is coming from, you can start troubleshooting. The most common cause of a parasitic drain is a faulty component in the electrical system that stays on even when the car is turned off. This could be something like a trunk light that doesn't turn off, or an aftermarket stereo that doesn't go into standby mode when the car is turned off. Another possibility is a short circuit somewhere in the electrical system.
To fix a parasitic drain, you need to find and repair the component that is causing it. If it's something like a trunk light, simply replacing the bulb may do the trick. If it's an aftermarket stereo, you may need to disconnect it entirely or wire it so that it goes into standby mode when the car is turned off. For a short circuit, you'll need to track down where the short is and repair it accordingly. This may require some trial and error, but eventually you should be able to fix your parasitic drain problem and get your car back to running properly again!