MSDN:
During installation, Extension Manager uncompresses the .vsix file and puts its contents in %LocalAppData%\Microsoft\VisualStudio\10.0\Extensions\Company\Product\Version. Company, Product, and Version are specified in the extension.vsixmanifest file, and correspond to the namespace, project name, and version number that are set in the project properties.
But strange, I also cannot find. I tried to install DbContextCSharp.vsix and find the content file (CSharpDbContext.Context.tt) of the installer. [You can unzip the installer by changing vsix to zip extension]
One thing I can suggest is, unzip the installer > update with your logic > change the extension back > re-install to visual studio.
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solved Where can I find the template files for project item templates I’ve added via Extension Manager in VS2010?