Ford has had a problem with the intake manifolds on these vehicles. They actually had a recall on them back in 1999. Unfortunately though the recall ran out seven years after the vehicle was put in service so for your car that would have been around 2004. The manifolds were not built henough to withstand heating and cooling. They started to fail and leak coolant in the front collant runner. I will attach the recall and a technical service bulletin for you to read. The Ford intake manifold is very expensive, but you can get one from NAPA for around $400. Labor time is around 5.5 hours at whatever the shops hourly rate is.
COOLANT - COOLANT SEEPAGE AT INTAKE
MANIFOLD CROSS-OVER - VEHICLES EQUIPPED
WITH 4.6L 2V ENGINE ONLY
ENGINE - 4.6L 2V - COOLANT SEEPAGE AT INTAKE MANIFOLD
1996-2001 CROWN VICTORIA, MUSTANG
1996-2001 TOWN CAR
1996-2001 GRAND MARQUIS
Some vehicles may exhibit an Intake Manifold crossover (first runner) coolant seepage condition. This may be caused by a crack in the Intake Manifold coolant crossover.
Inspect the suspect Intake Manifold for a coolant leak at the first runner cross-over area (Figure 1). If coolant seepage is found in this area, order the appropriate Service Kit and install parts, by referring to the appropriate Model Year Workshop Manual (Section 303-01).
4.6L 2V INTAKE MANIFOLD
Refer to Figure 1 for inspection of coolant seepage.
If seepage is found, order one of the Service Kits to repair.