You're looking for two switches/circuits.
The "door is open" circuit, which closes/energizes when a front door is open. The main purpose of this circuit is to light the dome light.
The "Park/neutral safety switch" which (hopefully) opens when the car is in park or neutral. This is normally an interlock with the starter motor to keep you from cranking the engine in drive. You may get lucky and also find a "Park safety switch".
If you're very lucky, the park/neutral switch operates by grounding out its single wire. Also look for extra unused contacts on the park/neutral switch. The goal is to find a switch which grounds out *when the car is NOT in park.
You are more likely to find a "ground if parking brake on" switch on the parking brake. Check its action: if it has unused contacts, you may find it has a "ground if parking brake off" mode.
If all else fails, get a limit switch and mount it appropriately. It will also work to use a magnetic reed switch and glue a neo magnet to the correct location so the reed switch opens in Park.
This makes it a simple matter of a diode and a buzzer of choice. The diode is to prevent unwanted backfeeding.
When you open the door, the "open door" circuit is energized. Current flows from this source, through your diode, through your buzzer, through the not-in-park switch if closed, to ground.