WCF error: extension could not be loaded

Change your type definition. First, specify the full type name (namespace + class name). After a comma, place the name of the DLL holding your type. And then the rest for a fully qualified type name. Like this:

<behaviorExtensions>
    <add name="customHeaders" type="InMotionGIT_NT.Address.Service.CustomHeaders, <DLLName> , Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
</behaviorExtensions>