OK, so at last I finally got around to fitting a challenge grill to the 360. Heres some instructions I found on another site, which were very helpful to say the least.
And heres a link to some other good advice, that also includes pictures. Ferrari 360 Rear Challenge Grill « Aldous Voice
Taken from another site.... Installing Challenge Grill on a 360
1. Mark the top of all the tail lights with the masking tape, just to ensure you install them correctly again. It may be a good idea to label each tail light with a number so you don't get them mixed up.
The idea to mark the tail lights with tape was a great tip. I marked the top of mine and also numbered them #1, #2, #3, #4. At first, I thought I could do the job without removing the tail lights until after the grill was out. However, I quickly found out you need access to some of the nuts which hold the grill on and the only way to gain access to them is through the tail light holes.
2. Remove harness from the back of each tail light, there is a clip that you will have to pull up, after that it will simply slide off.
3. Remove the two screws and washers holding each tail light off. After this, the tail light should come right off. Proceed to the next step once all four tail lights have been removed. Note that the brackets that come off holding the tail light harness will be used later.
4. There are 12 10mm nuts there are 4 by the hinge and there are 2 on each side of the hinge that are embedded into the bracing. At the bottom of the bracings there is a hole just in case a nut falls. There is one on each side of the top far corner of the grills and two more in between the ones on the top far corner and the two on the side of the hinges.
5. Now the OE grill is ready to remove, there are two zipties holding the tail light harnesses to the grill, you will need one person to hold the grill while the other cuts the harnesses free.
6. Now remove the OE grill, make sure to lay it down on a towel. There are two 6mm nuts holding the Cavallino in place. Remove these and the Cavallino will come right off, note that it will be very tight in place so try to wiggle it out without scratching the paint on the OE grill.
7. Put the emblem onto the new grill, your Challenge grill should have holes to put the Cavallino back on, again this is a very tight fit.
8. When installing the grill make sure to put two washers on the top two screws of the hinge, otherwise the hatch will not close properly. We ran into this problem.
9. Place a towel across the top edges of the bumper to prevent the Challenge grill from scratching the bottom of the bumper. Two people are recommended for this part.
10. Put the grill into place and note that it may be a tight fit so you will need to start tightening some of the nuts to pull the grill into place and the rest of the holes will start allign. Two top screws on the outer edge of the grill are recommended to get first, because it will bow out and try to rub the bumper otherwise.
11. Start putting all 12 nuts back on and tighten securly.
12. Put the brackets, bolts, and washers back on the tail lights. Make sure that the masking tape is in the same position it was when the tail lights were removed. Do this for all four tail lights until they are all tight and fit.
13. Clip the harnesses back into place, the two middle lights, the harnesses are yellow, and the outer lights, the harness is black.
Your grill should be back on, make sure your hatch closes and start the car up to make sure all lights are working. This should take 1-1.5 hours. It is a fairly simple job and the challenge grill is recommended for 360 owners as it's a way to help keep your 360 cool and boy does it look good.
Putting the tape on the bumper saved several nicks in the paint for sure. In fact, I would recommend putting two layers of the blue 3M painters tape. It's a big help when getting the challenge grill on, especially on the corners.
On my factory rear grill (the solid one), there were two washers on the top two screws (the ones on either side of the latch in the middle). There were actually 4 total, but I didn't end up using the other two (which I think were in each corner, but not sure).
Some of the 10mm nuts are difficult to remove. It takes a few different tools. One tool I found really helpful was one of those little round drives that you stick a socket on. It sort of looks like a wheel and is only about 3/8" thick. That was a big help. They are included in the Metrinch tool kits luckily (see http://www.mitools.com/
Installing the challenge grill is a little tricky. It's not a perfect fit, you sort of have to force it into place, then slowly pull it tight with the bolts. At first, I thought I had run into some real problems, but as I tightened everything down, it snugged the brackets up to the car. When all was said and done, it fit like a glove.
The clearance between the bottom of the challenge grill and the top of the bumper is very tight. When I was all done and pulled the tape off, it was nearly an interference fit right below the latch area (in the middle). The rest was about a 3/32 to 1/8th gap or so. Again, using the tape was very important I thought! And certainly saved me a couple of nasty scratches in the paint.
Once installed, check the engine lid to make sure it latches. I also ended up unscrewing the rubber stoppers about 1/2 turn on each side so that the lid would sit down properly and give a little "pop" when you open the latch. Don't go crazy though :-)