I connected it to a switched circuit because the manual said it could discharge the battery.
Christian. I was a bit unsure about that myself but the one I have fitted is quoted to have a parasitic current draw of 15mA and I figured that, with existing parasitic drain, the car should still start OK after at least 10 days if not two weeks. I use a battery conditioner so I thought it would be ok and I wanted it to function when the car was not running. In hind sight perhaps it would have been better to wire it in to a circuit that was live when the key was in the accessories position.
If it presents a problem I can always change it.