This information was requested by Conceyted and I thought I would post it here for others benefit.
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The two diffuser part numbers are included in the following information given to me (thanks!) by reenmachine and ChinaClipper. Note one side takes two nuts and the other side two bolts. These diffusers are black plastic and the left and right are quite different in shape. It took me about 10 minutes a side to install them. You have to drill one hole on each side of the existing plastic inner fender liner for one of the clips. It will be obvious where to drill once you figure out how the diffusers fit in behind the wheel. I installed mine without removing the rear wheels or raising the rear.
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Author:_ reenmachine _(---.242.8.67.cfl.rr.com)

Required stuff (according to "A17Z"):

Driver side
2 - 10mm nuts (sparky3Z: this is the wrench size, NOT the hole size—I think it was 6mm but it may have been 5mm. You can check before buying.)
2 - plastic fasteners (clips)
1 - rear driver side diffuser 748A0 CD810

Passenger side
2 - 10mm bolts (sparky3Z: this is the wrench size, NOT the bolt diameter—I think it was 6mm but it may have been 5mm. You can check before buying.)
2 - plastic fasteners (clips)
1 - rear passenger side diffuser 748A0 CD800

Threads:

https://my350z.com/forum/showthread....track+diffuser

https://my350z.com/forum/showthread....track+diffuser

https://my350z.com/forum/showthread....track+diffuser

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The following photo and comments are by ChinaClipper:

Shot of the Left rear diffuser.

This shot is taken from directly behind and underneath the car, looking up and forward towards the left rear wheel well with the tire and wheel removed.

 

I bought mine from:

http://www.courtesyparts.com/

 

Price?

 

The two diffusers were $28.99 each ($35.83 list) and the clips were $2.52 for four. The part number for the clips is 01553-09321.

 

i appreciate it! thank you