If we had a tall mountain nearby and an adapter that allowed

Solution

Ans:Say the mountain in very tall it is in a frame which is at rest with respect to the earth frame.Now the car is moving at a relativistic velocity.The detector is owered up through the 12v plug in ina the car.So the detector consumed energy from the car battery.
Now the detector maker or we know what is the power rating of the detector i.e how much energy it will take in a certain time intervel.Accordingly we know what is the rate of power consumed of the detector when it is at rest.

Now in the earth frame or in the frame where the car is crossing the mountain and the mountain is at rest.we can calculate the time taken by the car to cross the mountain and it is (mountain height)/(car speed).So we know what is the power consumed by the detectr during this time.
Now if the detector is inside the car we can see that during this mountain crossing time the power consumed by the detector from the car battery is higher than that if the detector is in the earth.
So by measuring this extra power consumption by the detector from the car battery we can tell that there is a time dilation.

Basically if the detector consumed higher power it the car battery voltage will go down.