You would have a leak, from either the pipes to the heater matrix (radiator), or the matrix itself.
heater core
cap is off
Could be anywhere along the radiator hose to the journal where the thermostat is located, the mounting plate that the oil filter is screwed on to.
Are you sure it is a antifreeze leak? If so ,stop leak will not help heater core leaks.But make sure that is the problem.
You have to determine the source of the leak then decide on how to fix it.
water pump
I am sure you have a v8 engine----if so the water pump weep hole will leak antifreeze and show up running down the lower rad hose on the water pump side ---of course there are other places antifreeze can leak at the front of the engine such as the water pump mounting gasket----the lower left (drivers side) p ump mounting bolt---as it enters the water jacket--- the timing caver gasket and the intake manifold gasket but the pump is the most likely place
windshield leak that runs down behind the dash and onto the floor.
Heater core leaking?
I had a 2000 Lincoln LS and the Antifreeze Reservoir had a rash of heat induced cracks on the side facing the Firewall. The antifreeze would ooze and evaporate without leaving a puddle but a whiff of the smell could be noticed on some occasions. My 2005 LS has some of the same cracks but has not started to leak or ooze antifreeze. Also on my 2005 LS there was an upper hose leak but only with 16 psi or more of pressure.
If it comes from the side of the motor and not from another source such a a leaing hose, then it is almost deffinatly a leaking head gasket. not a cheap or easy fix.
Heater hose or heater core.