this is easy and inexpensive wireless 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!
In attached files to be printed in 3d for RX and TX case
PCB module is editable with fritzing
you can make this single face PCB with cnc or transfer method
attiny85 is easy to program with arduino nano or uno with this guide
RX/TX modules are powered with 9v battery but you can use any DC power with 5v to 20v voltage without problem.
For car alarm i suggest to use car inside lights to be activated with the opening of the doors.
this module can send code of alarm so it can be used as alarm for the house by inserting it in the doors, windows or putting in series with sensors (gas, fire, infrared)