how do i set the interior lights to go on automatically when the door opens?

Check dimmer switch. On some vehicles when dash lights are dimmed to their lowest position the interior lights will not come on when doors are opened.

If that interior light is still not fixed perhaps it could be a computer module problem, btw you should still be covered by Toyota warrenty have them check it out for free.

This seems silly, but it took me 30 min to figure out that there are 2 switches that control the interior lights coming on when the door is open. The obvious one is in the complete center of the car (sort of over the driver's right shoulder - and this one should be set to "door" But it still won't work UNLESS the front-top light switch (sort of above the rear- view mirror) is also set to DOOR.

Jvalli76's answer was spot on and just helped me 4 years after it was written on my gen 1 Prius. The top done it and passenger dome lt didn't work since purchased it 2nd hand, but the driver dome lt is sufficient so only checked at nite when it stopped coming on when open door. Never looked at top center switch til read Jvalli's post. Very hard to read lables molded into the translucent plastic, but with high intensity lite off to side saw rear sw POS said door. Moving lever switch to rear (door POS) fixed puzzeling problem. Maybe some day I'll replace the bulbs in passenger and center dome, for 100% fully working dome lts, but the good bulb cones on now when needed, so I'm good for now.