Instead of the radar motion detector which could be detected by thieves and attacked by interference, find a PIR motion detector which passively detects moving humans up to 30 meters away.
PIR security detectors are low-cost and several versions are sold as parts controlling spot-lights. I used such devices for years without any problem and thieves knew that they were detected as the light was coming to their faces.
The PIR sensors can sense also movement of animals and larger objects, so locate the sensor so it can only see your car side where tank is. You can add a camera to record their shocked faces, too.