If your 1999 Ford Crown Victoria has an antifreeze leak at the rear of the engine what could it be?

check your heater hoses one connects to the intake on rear passanger side of the motor it is likely the intake cracked there is a recall on this issue if the intake has been replaced with factory intake it is possible that the intake gasket is leaking check the bolts to be tourqed to specs if gasket is leaking replace gasket

Crown Victoria Plastic Intake Manifold Heater Leak Fix

When the stupidly designed intake manifold cracks where the heater pipe sticks into the plastic here's how to fix it. You will notice the plastic around the heater pipe is connected to some plastic to the front of the car from it. Take a hacksaw blade and cut about a 1/2" groove in that plastic - just enough to get a hose clamp around the plastic circle that holds the pipe. Tighten the clamp and voila! No more leak.