I would have to disagree with your belief that bathing is not instinctual, but learned. I base my position on the fact that I have raised many parrots that were incubator hatched, or hand-raised from just hours or days old when parent-hatched. Virtually all took to bathing , without prompting or instruction about the same time they started to fly. Like flying and feather preening, I think bathing is instinctual, and birds will do these things even they have never been in contact with another bird.
When raising parrots in a home environment, we have to provide the proper environmental elements to stimulate the birds natural drive (and need) to bathe. Unfortunately, the "right conditions" are not always provided.
When my babies (from lovebirds to parrots and macaws) are ready to fledge, in the morning, I will provide an appropriate basin, or bowl of lukewarm water - and set this in a warm room. If possible near a sunny window, if not under a light -- then I back off a bit (close enough to supervise - but far off enough to not distract the bird), and stay still and quiet. Most start splashing about right away.
If its not possible to set-up a bath, you can introduce your bird to a spray bottle--again, a warm, bright setting is best. Start by misting lightly, from a distance to let the mist kind of "drift" over the bird - you can increase the volume of the spray as the bird responds positively. If you do this at a consistant time every other day, the bird will get used to it pretty fast -- in fact after a few of these "scheduled" shower sessions, you may find your bird "starting without you", splashing himself with water from his water bowl. Many birds soon start bathing as soon as they are given fresh water.