‚Äčthis is easy and inexpensive wifi alarm with 200m range if module is insert inside car or >500m if module is outside two modules: RX and TX receiver module make sound alarm and led blink when detect signal transmit module is connect to any device or switch to turn it on ¬† Hardware is arduino attiny85 and nrf24l01, very inexpensive!

you can make this single face PCB with cnc or transfer method

Receiver module is powered with 9v battery but you can use any DC power with 5v to 20v voltage without problem.

Transmit module can also powered with any voltage.
For car alarm i suggest to use car inside lights to be activated with the opening of the doors.