I read this article in Corral mustang forum. I copy and paste for you. Hope it help.GT Grille and Fog Light Conversion for the V6
I am not trying to advertise, but I need to address this issue. I have seen several people that have tried to jump on the GT Grille swap for the V6 including some Ford dealers. The GT Grille is taller and wider than the V6 as we all now. I have e-mailed the dealerships that I saw advertising this as though it was a direct swap. One did not replay and the other said, "you may need to switch the bumper cover, it is a different part number", end of replay. I e-mailed the guy and suggested that if that is what was needed that you should put that in your advertisement, I received no further reply, and no change was made in his advertisement. I would also like to say that Ford is not helpful when dealing with "parts swapers" as they call us. I also noticed on several forums, this one included that people are talking trash about the swap, or are saying that is can't be done. The GT grille is wider in the back than the V6 Grille. The taper of the sides on the V6 are greater than the GT…The GT is almost the same from front to back and the V6 is about a 1/2" smaller. The angle of the bumper cover side pillars is where the difficulty comes in for the swap. Saying all this, you can get the GT Grille to fit into the V6 bumper cover without modification to your bumper cover. I do sell a kit with all the parts, grille, lights, wiring harness, switch...the whole 9 yards, but you will have to find it if your are interested, I am just giving our experience with the grille swap here for those that have an interest in doing this to their Mustang. I wanted everyone to know what was entailed.
The GT Grille can be fit into the V6 bumper cover. You will need to unhook the bumper cover on both sides of the grille. Remove the V6 grille, starting at the top clips and working your way down. DO NOT DAMAGE THE RETAINER TABS on the bumper cover. Install your wiring harness and plug it into the fog lights. NOW HERE IS THE MOST DIFFICULT PART OF THE WHOLE DEAL.
Remove the bottom clips on the GT Grille, they don't line up, and they don't reach back far enough to clip into the bumper cover so you don't need them. Remove them with a pair of tin snips (JUST THE BOTTOM CLIPS ONLY). Clip the drivers side Grille clips into the bumper cover. NOW HERE IS WHERE YOU NEED TO DO SOME WORK AND HAVE SOME PATIENCE...Clip the passenger side in. This will require some muscling, perhaps some assistance with a screw driver to line the slot and the grille clip. Do this carefully, but it can be done. Once both side clips are done, us a pair of pliers and position the retainer tabs on the bumper cover into position as much as possible to hold the grille in the bumper cover. Replace the bumper cover to the original position, clipping both sides back into place and replacing the retainer bolts on both sides of the grille. Then reinstall the engine compartment (fan shroud) cover with the original 6 plugs, in the top of the GT Grille and radiator cross member, these holes will line up or can be made to line up.
The final result is that the GT grille will sit slightly farther out on the bottom than the stock grille, so the bottom of the grille sits out over where the bumper cover and the grille meet. Because of the lines of the bottom of the grille, the bumper cover and the hood, it looks all pretty stock. I have shown my car to a Ford Parts Manager and Service Manager at my local dealership and they thought it looked good. There is a slight gap between the bottom of the grille and the top of the bumper cover, again because of the lines, it all looks stock. Unless you had your car sitting next to a GT and pointed out the difference, it is unlikely that anyone will notice. No one has noticed on my car.
The lights issue. You cannot just change the switch and hope everything will work. The GT and the V6 have a different wiring harness under the hood. The switch in the dash is a low power switch that seems to operate relays to power the fog lights and head lights...so you can't just tap into the switch. If you look in your owners manual you won't even find a fuse or relay for the headlights, only the fog lights and the high beams (but fog lights aren't supported in the V6 Fuse box). Because of the relay system you don't have "old school" on/off power at the head lights and the wiring there is insuficient (we think) to run both the head lights and fog lights anyway. So the best route is to wire a seperate harness that isolates the Fog lights in it's own circuit. Requires no splicing and if you do it right, no one even notices it under the hood. We use a 15 amp in-line fuse to protect your electrical system. The Fog Light bulbs are a new (of course, Ford has to switch things up all the time and use different plugs to keep things interesting) Sylvania #9145 or H10, and you can get the plug with pig tails, to plug right into the fog lights from your Ford Dealer to make the Fog Light end of your harness, if you are going to build it yourself. The motorcraft part number for the plug with pig tails is 1U2Z-14S411-FA and can be obtained through your Ford Dealer.
The bottom line is that this swap is possible, I know cause I have done it and now we have several customers that have done it. It looks great! But BEWARE OF THE PARTS GUY OR AFTERMARKET GUY ARE JUST TRYING TO THROW SOMETHING TOGETHER TO GET SOME SALES AND HAVEN'T DONE THEIR HOME WORK TO MAKE IT WORK RIGHT FOR YOU. You don't need to start paying through the nose, cutting up your nice new Mustang...shop around. You can find a lot of the parts listed here and there on the Internet. Or you can get most of the stuff I talked about here from your local Ford Dealership. But that is probably the most expensive place to buy it.
Hope this helps | 
2005 Mustang GT Grill on a 2005 V6 mustang 
