Residual Voltage Causing Bootloop

[Doc]

Today I hardwired my dashcam and radar detector in a 2017 Nissan Altima I used a fuse tap device. When I plugged in the fuse tap and turned on the key, voltage tests good on my meter. But, when I turn off the ignition, the voltage drops to ~5 volts and slowly goes to zero after a minute or so. I tried a couple of other fuse slots and it is the same.
This is not an issue for Uniden R3 radar detector. But it is for a G1W dashcam. It keeps trying turn on but shuts down. It keeps doing this until the voltage goes low or zero. It's like a boot loop.
My other cars do not have residual voltage. So the G1W dashcams work fine in those. I asked at the stereo shop today. They don't know of any fix. He said some cars do that.
Anything that can be done to help? I can turn off the cam before turn off the key, but....I shouldn't have to do that.

 

[jokiin]

did you try just the dashcam without the radar detector connected?

 

[Doc]

I have not. But the voltage is there with just a meter connected. I was just testing the tap before I connected anything. As soon as I turned off the key, voltage drops from 12V to 5V and slowly drops to zero. Never seen that before on my other 3 cars. It definitely affects the dashcam.

 

[jokiin]

if your radar detector has a battery inside it could be feeding power back, being that it's 5v you're seeing that's a possibility

 

[Doc]

There is no battery in the RD. The 5V drop is coming from the car. This is with nothing connected. Just a volt meter.

 

[jokiin]

detector got capacitors inside? would be very unusual for 5v to be present anywhere in the 12v system, sounds more like an accessory that is feeding back, what else is on those circuits?