Haven't changed any bulbs since I bought the car 2 years ago. As for when it comes on: it sometimes (but not always) comes on when I first start the car (whether the lights are on or not). If it's not on at startup, I can turn on the headlights and tap the brake pedal and it still won't turn on. Then it seems to come on either when I first move the transmission lever (I mean like a half inch, to get ready to pull it down into reverse), or just on its own after a couple of seconds, I'm not sure. Anyway, I may have time today to take a closer look at some of the bulbs, so I'll start there. Maybe something's a little corroded and adding resistance.....
---Ethan