I think that 5.45 is a reasonable time to start with, although you will probably want to move it forward quite quickly once he gets the idea of the lights - if you keep it too early for too long then it gets quite difficult to move them on from it.
If he wakes and calls you, then definitely go to him - tell him that it's not morning because the light isn't on and he needs to go back to sleep. If you want to stay in the room with him then that is okay for a few days - keep telling him that it is still sleep time because the light is off and then when it comes on make a big deal out of "look! The light's on - it's morning! Time to get up!" so that he really hears the difference. I think you should only do this for a couple of days, though, otherwise you will just end up stuck in his room from whenever he wakes up. With my children they pointed to the light when it came on on the third day, so they clearly made the association that quickly (they were about 17/18 months when we started using it) - after that if they woke before the light then I did wiwo rather than staying with them.
As for the nap in the car, I don't think there is much you can do about it. Hopefully the lights will help push his wu a bit later and he won't need that nap any more.