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 #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.