Wiring constant fan control furnace

I an trying to add a fan control switch on my furnace (A Trane TR90) either as part of my thermostat, or separate, since this furnace does not have a plenum switch I can switch on to run this to circulate air in the house during the summer. Anyone have any ideas on how to wire this? I have been looking at the diagram, but since it is a mid/high efficiency type they do not have the controls on it that I am used to. The furnace connections are : R Y G W, and only R and G are connected.

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