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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.