Rails 3 - Custom Validation

I'm somewhat confused by my options for custom validations in Rails 3, and i'm hoping that someone can point me in the direction of a resource that can help with my current issue.

I currently have 3 models, vehicle, trim and model_year. They look as follows:

class Vehicle < ActiveRecord::Base
  attr_accessible :make_id, :model_id, :trim_id, :model_year_id
  belongs_to :trim
  belongs_to :model_year
end


class ModelYear < ActiveRecord::Base
  attr_accessible :value
  has_many :model_year_trims
  has_many :trims, :through => :model_year_trims
end

class Trim < ActiveRecord::Base
  attr_accessible :value, :model_id
  has_many :vehicles
  has_many :model_year_trims
  has_many :model_years, :through => :model_year_trims
end

My query is this - when I am creating a vehicle, how can I ensure that the model_year that is selected is valid for the trim (and vice versa)?