First of all, you do NOT always have an obligation to retreat. This varies widely by state. This is where it is critical that you understand the laws in your area.
That said, it is always best to retreat if you can do it safely.
In this situation, as described, I hope that I would:
- Put the car in gear (forward or reverse) and leave or be prepared to leave.
- Call 911.
- Leave the windows rolled up.
- Keep my cool.
- Have my handgun readily accessible, but not necessarily in my hand.
Remember, that if you are in your vehicle, it makes a tremendous weapon in itself if the aggressor draws a weapon of any type.