Camera surveillance for the trunk?

I'm planning a longer drive with my family in the next few weeks.

In order to be able to monitor the health of our sick dog while driving, I'm considering installing a camera in the trunk. However, when I look for cheap options on the Internet, I only find surveillance cameras for the house that work via Wi-Fi. However, I don't feel like spending hours on my data volume on a Wi-Fi in the car, so that's not an option.

I would like to avoid a webcam because I don't want to have the laptop on my lap all the time.

A small mobile camera that sends a live image to my smartphone (Android), if necessary by cable, would be ideal. Does anyone know about it and have a tip for me?

You can buy a reversing camera, but not outside, but mount it in the car.

Clever!