The way we have done it is to remove the assmebly, and drill through just the fresh air duct inlet plastic welds, and remove just that piece and it's blend motor, then seal off the fresh air intake on the underneath of the cowl with a piece of sheetmetal and you are done. This way the heat and air both work on all speeds correctly and it doesn't freeze up, it's just on permanent recirculate which is where I always run mine anyway. You don't have to do any other mods to the under dash box, also this will clear pretty large tubs, not just small tubs.