Activating the REST function (when conditions permit its activation), is also a good way to test the auxiliary water pump's function.

This auxiliary water pump is used to circulate more coolant thru the heater core than would otherwise flow without it under some circumstances.

If the auxiliary water pump is not functioning correctly, you'll only have around 30 seconds to a minute of hot air after activating REST (Assuming engine coolant is fully warmed). With a properly functioning auxiliary water pump, you should have at least 5-10 minutes of pretty good heat coming from the vents that trails off to lukewarm in about 15 minutes, as this pump is used to continuously circulate hot coolant when using REST.