really loud AP outside
Is of no help as said. (Wifi is always bi-directional communication)
What does help is a device with high antenna gain. (But! Antenna gain when properly configured will reduce the TX power , to again send within the legal limits, now only in a segment, nice for the neighbors, and your patio)
So no real gain ? Not for TX power, in transmitted signal, it is not louder and not better.
Being 'louder' in wifi makes not much sense. (Wifi is like a "polite" meeting, if anyone speaks, even whispers, you shut up).
But all the gain will be in the receiving of the cars wifi signal!
e.g. If you have a SXTsa5 or similar, the gain in receive sensitivity is 16dB. You will receive the car signal 14 dB stronger, than a standard 2dBi dipole antenna.
How to make a long distance connection? Well both sides of the link need high antenna gain.
No idea if the car has antenna options.
If you have "keyless entry" on your car, this a-symmetry is there. The key-fob signal is very strong (100's of meters of range) , the car signal is extremely weak (70 centimeters range). And that car signal is used to detect if you are near your car. If you are , the key-fob will send an authorisation, doors open and the car can start. Thieves amplify or repeat your cars signal, and are able to drive away, if their (repeated) signal reaches the key fob.
No breaking in, nothing taken, key fob still in your bedroom. Car gone.