904 Leaking. What to check first?

Agreed on selector shaft. Your backup light / neutral safety switch is soaked, which tells me its coming from there. Seal can be replaced in car but it can be a real chore. Also check your dipstick tube, a simple 3/4" (?) plumbing o-ring will fix that right up.

Couple other places to check are speedo and there is a small cover plate beneath the cross member that can weep. Its for accessing the snap ring to pull the yoke.