The first thing to say here is ... mod files at your own risk !
I'm no expert but I get by ... :D
To mod GRID's decals etc ... ...
First ... you need some knowledge of DDS files, alpha channels and Photoshop ... I can't teach you this.
You will need this tool here ...
GJ_PSSG_Utility ... from gridjoint.com
What this tool does is extract the .DDS files from the games .pssg files ... which are the various decals in the game.
The .DDs files can then be edited in Photoshop (or Paint Shop Pro I believe).
I use Photoshop.
The tool also makes a backup of the original files which you can restore at any time.
Esentially what you do is edit one of the in game sponsors to be something you want ...
I changed the COBRA logo ... to the picture of Paddy with the word Racing ...
Screenshots below ...
Run the tool, and click on the "Browse" button.
Select a PSSG file ... my example ... C:Program FilesCodemastersGRIDcarsuser_liveriessponsorscobra_1.pssg
Click on the "Extract" button ...
A new folder called "C:Program FilesCodemastersGRIDcarsuser_liveriessponsorscobra_1_pssgfiles" will be created.
Navigate to that folder on your comp and you will see the DDS files for that sponsor.
In this case just one file ... cobra_1.dds
Use Photoshop to modify it ...
You may need a plugin to do this.
The Photoshop DDS Plugin is here ... http://developer.nvidia.com/object/photoshop_dds_plugins.html#downloads
Once you have done the changes you desire save sponsor decals with these DDS settings ...
DXT5 ARGB (Interpolated Alpha) generate mip maps ALL
Now go back to the tool and click on the "Import" button.
Once done ...
Play the game ... apply the sponsor decal (that you modded) to your car (in the case above - COBRA) and your modified decal will show up not the COBRA logo.
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To edit/modify the window or tyre graphics it is a bit different ...
These are specific to the cars and specific to the user.
In my case I have modded the Viper window and and tyres ...
Using the tool ... navigate to ... C:Program FilesCodemastersGRIDcarsviplivery_usertextures_high
Click on the "Extract" button ...
A new folder will be made ... C:Program FilesCodemastersGRIDcarsviplivery_usertextures_highvip_tex_high_user_pssgfiles
It has all that cars DDS files ...
Edit the DDS file of your choosing ...
Once you have done the changes you desire save window or tyre DDS files with these DDS settings ...
DXT5 ARGB (Interpolated Alpha) generate mip maps ALL.
Now go back to the tool and click on the "Import" button.
Wait for it to finish.
Play the game ... your new window or tyres should show up on
that car (your car) in the came.
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Please feel free ask questions ... it I know the answer I will try to help.
Also correct me if I have anything wrong ... as I say I am no expert.
Some shots ...