Mike,
I have one of these about to be fitted.
Having tried it just in the lighter socket, it appears that the lights come on with the accessories, ie, when it's powered from the vehicle rather than the internal batteries, therefore no button to press.
I have rewired it to give a direct feed from wiring rather than from the lighter socket and re-exited the wiring from the rear, rather than the socket on the side. I'm going to position it below the radio and find a power feed in there somewhere and then try and find the best route for the external temperature sensor. (When it stops raining)
That's if I haven't dropped a blob of solder inside and kyboshed the whole thing
Pete.