The tee stays in the line, it just provides a place to connect the garden hose source of fresh water for the flush ( and has a cap).
So you don't replace the heater hose, only modify it. I think on my 3.0 I put the tee instead of the heater control valve as a temporary connection, then put the control valve back in when I was done with the flush. The kit is generic, not all vehicles have a spot for a temporary tee installation.
There is a back flow preventer in the kit to keep any coolant from going back into the garden hose connection. The kit was cheap, it's only a bunch of plastic parts (and not many parts, either).
1998 Ranger reg cab long bed XLT 4wd 3.0L 5 speed manual 4 whl ABS option, Delta toolbox, Pioneer DEH-3300 w/USB, deep jewel green exterior/medium prairie tan interior. Goodyear Duratrac 245/75R16. 165K miles. Amateur radio N1EWB.