Zetec has the PCV connect to the back of the intake manifold. The breather valve on the Zetec valve cover goes to the air box so its port is after the stock filter and before the MAF.
There is a "special" yellow foam filter in the air box about 1" square specifically for this.
Most aftermarke air intake systems will pull this off and either provide or require a second breather filter. Ideally you would still want some vacuum on the line like it would see in the stock configuration, but is not needed.
The main reason for this is because emissions usually require any "vapor" from the motor, must go through a "device".... these devices on a focus (and most cars) are the catalytic converters. Routing the "vapor" through the intake is the answer to this problem. but once you get miles on it usually it just mean that its coating your intake with oil because manufactures usually skimp on having a good oil/air separator installed.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)