Hi there and welcome
It sounds like it is connected (the camera) to your battery and not the brakelights (which means it only comes on when the vehicles in reverse).
But when the camera is on, press the return arrow on the screen and this then turns it off.
If you press this, does it then turn off? To be helpful I show you the icon in the manual on page 3 here
Let me know please.
ok, that is not normal behavior. Has it done this from the start (as soon as it was installed)?
ok, either the installation was not done correctly and its cross circuiting that keeps making it start or the Garmin itself is faulty.
Do a factory reset which will remove all data and return the unit to new. To do this, press and hold the lower right corner of the screen and then power the unit on, a dialog will come up asking if you want to "Erase all user data", select yes.
Once done, check if it still has the issue, is so, you will need to have someone to check the installation. Even though it was professionally done, mistakes can still be made.
If the installation is ok, then the unit itself has faulted.
In this case you are going to have to return the Gamin to have it replaced. It will be covered by the warranty if this has not expired, and the replacement will be done at no cost to you.
If you have problems, or would like any additional information or assistance, please do not hesitate to let me know.
And if you can take a second to rate my service, that would be greatly appreciated.