On my 70 Monte Carlo with idiot lights my temp light does not light up when the key is on but it does light up in the start position. I installed a new temp switch and i'm getting 12v at the green wire terminal with key on, if i disconnect the green wire and manually ground it it will turn on temp light. Is the temp light supposed to be on with key on and in start or only in the start position?

I think it is only supposed to come on when you turn the key to start the car or ,of course, when the temp. is actually high. That's the way it works on my '70 Chevelle.

Is the temp light supposed to be on with key on and in start or only in the start position?

Only in the start position.