You really are going to have to do some detective work and find out where is coming from. Plastic intakes are notorious for leaking, could be a hose, the coolant tube under the intake, water pump, a headgasket etc...
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.