Arduino sleep mode - Wake up by Rx pin from SIM module?

Hello, i have an arduino project where i start my parking heater on my car via can-bus by sending an sms to a SIM800L module.

The SIM Rx and Tx pins is connected to arudinos i/o pins 7 & 8.

The current withdrawal right now for all devices is 35mA on 12v. My worries is that it will drain the battery if the car is unused for a long time.

I played around with different arduino deep sleep libraries and got the current down to around 20mA, but i could not wake it up. Can you guys guide me how it should be done?

I guess the first message from the SIM will be discarded while the arduino is waking up, but that is no problem, i can send 2 messages.

I guess you’re using that horrible SoftwareSerial, so the Rx pin is activated to use a pin change interrupt. Your Arduino should wake up as soon as serial data arrives. My guess is an error in the code you use to put it to sleep. Post your complete code and post a wiring diagram!

35mA idling is reasonable, but the modem will draw lots more at times while the cpu is asleep.
This could become a moot exercise.