Basically removing your upper grill at all can be problematic. But don't let it put you off.
The cause of the problem is simply when you remove the key barrel from the existing grill and let the barrel hang down, the weight of it will dislodge the connection of the lock mechanism.
So when you've removed the old grill, fitted the new one, put the barrel in the new grill and closed the bonnet you will find it won't open and turning the key does nothing.
So to easily avoid the problem in the first place, don't let the barrel hang and balance it on an object to keep it in position.
But then even if you have let the barrel hang and you get locked out your bonnet, it's really easily fixed.
I've written a guide explaining the problem in detail and how to fix it easily. I know it's already helped a few members in the same situation. Including Rh1no who owns a zetec s and fitted the ST grill so he'll tell you how easy it was to fix.
Thanks, do u know anywhere that sells replacement front and lower grills for zetec s, i did find a website but then lost it and for the life of me can not rememeber what it was