I have a Grand Prix, but no schematic. Your 500 point relay seems to be remaining
energized, or at least in the energized position. The 500 point relay is actually
under the playfield, not on the bottom board. It's the relay right next to
the right inlane's bulb that states "Extra Ball When Lit"
Not sure if you had, but I'd check the contacts in that relay. Maybe they got
bumped pretty close together, and any vibration makes it energize itself.
There's 3 sets of contacts in the 500 point relay. The lower set is primarily
the one you want to make sure aren't too close together..
I believe there's only 2 playfield switches that activate the 500 point relay.
One on the left saying "Advance Left Bonus 500", and one on the right saying
"Advance Right Bonus 500". It sounds like you have other coils staying fired,
but going by your video, I'd recheck the 500 point relay and the two 500 point
switches (and their solder lugs) on the playfield.. (T)