Applying .tt on build using T4ToolBox

Question

I have got some .tt files in my solution for a long time. I recently decided to launch the transformation at build time by adding to my csproj :

<PropertyGroup>
    <TransformOnBuild>true</TransformOnBuild>
    <TransformOutOfDateOnly>false</TransformOutOfDateOnly>  
</PropertyGroup>  

I have a first .tt file (First.tt) which includes a second .tt file (Second.tt) from another project of the same solution:

<#@ include file="..\SecondProject\Second.tt" #>

In Second.tt, I include T4Toolbox.tt :

<#@ include file="T4Toolbox.tt" #>

Everything is ok when I launch First.tt by using right click -> run custom tool. But when I rebuild the solution, I obtain this error : Failed to resolve include text for file:E:\Dev\MySolution\FirstProject..\SecondProject\T4Toolbox.tt

So I try to use the installation path of T4ToolBox.tt :

 <#@ include file="C:\Program Files\T4 Toolbox\T4Toolbox.tt" #>

But I have got another error : A processor named 'T4Toolbox.DteProcessor' could not be found for the directive named 'dte'

If anybody can help, it would be great !

EDIT : I have not succeeded but it is better. I removed the rooted path in Second.tt and I have added the following lines into the csproj of FirstProject :

<PropertyGroup>    
    <IncludeFolders>C:\Program Files\T4 Toolbox</IncludeFolders>
</PropertyGroup>
<DirectiveProcessor Include="T4Toolbox.DteProcessor" >
    <Class>T4Toolbox.DteProcessor</Class>
    <CodeBase>C:\Program Files\T4 Toolbox\Bin\T4Toolbox.10.0.dll</CodeBase>
</DirectiveProcessor>
<DirectiveProcessor Include="T4Toolbox.TransformationContextProcessor" >
    <Class>T4Toolbox.TransformationContextProcessor</Class>
    <CodeBase>C:\Program Files\T4 Toolbox\Bin\T4Toolbox.10.0.dll</CodeBase>
</DirectiveProcessor>   

When I try to build, the dte processor is found but I have still an error : An Exception was thrown while running the transformation code. The process cannot continue. The following Exception was thrown:

System.NullReferenceException: Object reference not set to an instance of an object.
   at T4Toolbox.DteProcessor.UpdateFiles(ICollection`1 outputFiles)
   at T4Toolbox.OutputManager.UpdateFiles()
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at T4Toolbox.TransformationContext.OnTransformationEnded(TextTransformation transformation)
   at Microsoft.VisualStudio.TextTemplating7C4960D0E15B020F44790E9542C211BF.GeneratedTextTransformation.Dispose(Boolean disposing) in E:\Dev\MySolution\FirstProject\First.tt:line 18
   at Microsoft.VisualStudio.TextTemplating.TextTransformation.Dispose()
   at Microsoft.VisualStudio.TextTemplating.TransformationRunner.RunTransformation(TemplateProcessingSession session, String source, ITextTemplatingEngineHost host, String& result). Line=0, Column=0  E:\Dev\Tools\Microsoft\Microsoft.TextTemplating.targets 397 5   FirstProject