I bought a heat only unit from proAir LLC... it stands up vertically inside the driver heater box.. then I used automotive flex dict and tied it into the OEM defroster vents..
I had taken my driver heater console out and Gutted it completely.. I placed a pic but you can really only see the combo Heater and A/C unit (on the left).. my heat only for the defroster stands up on the right side of the box.. I was able to connect its intake to the OEM outside fresh air intake so the defroster gets fresh instead of recirc air....
others are made by red-dot, transair, and then jegs and summit sell heat only units you could leverage for the defrosters.. the OEM vents on my bus are placed decently so I left them intact and just replaced the heater itself..