Smell is usually the # 1 kicker... The leaks can be very small but when its heated and blown the smell does get stronger, even if its tiny its going to need to be replaced. Another common show point is if equiped (rear vents that run through console) the leak will chase the vent tube. I opened mine up on the passenger side beneath the carpet on the transmission hump nearest the dash. On my 99 i have a stock drain that showed me on the firewall behind the engine more prominate. Picts below
(from fenderwell-location behind trans dip)

Top engine bay view