Cannot open .mtl file unless path is hard coded

Question

I'm having trouble with my .OBJ file model loader. I am reading in the model fine, but when it comes to reading in the .mtl file the file will only open if I hard code the path.

I have check the path being given, and it matches the path I hard coded (which works just fine). But when ever I try to have the code either read the path from the .obj or just build it in the code, the file won't open.

Bellow is the relevant code. If I have missed anything, let me know and I will provide it.

bool ObjLoader::LoadOBJ(const std::string& filename, 
                        const std::string& filePath, 
                        std::vector<Vertex::PosNormal>& vertices,
                        std::vector<USHORT>& indices,
                        std::vector<MeshGeometry::Subset>& subsets,
                        std::vector<OBJMaterial>& mats)
{
    std::wifstream fin (filename.c_str());  // <-- .obj path

    if (fin)
    {
        while (fin)
        {
           // reads in .obj file
           // works just fine
           // meshMatLib read in from here                                     
        }
    }

    // Close the obj file
    fin.close();
    fin.clear();

    fin.open(meshMatLib);  // <-- uses hardcoded path above for /mtl

    if (fin)    // if the material file is open
    {
        while (fin)
        {
            // loads Mtl file
            // will not open
        }
    }
}

The below line is from the .obj file and is used for the .mtl path. mtllib Models\testTeapot\testTeapot.mtl

the hard coded path which works is:

 "Models\\testTeapot\\testTeapot.mtl"

It is passed into the method when LoadOBJ is called ('filePath')

The fact that this works when I hard code the path, suggests to me that the issue is related to the path. Is there a way I can get any further information on why the std::wifstream has failed?

Thanks in advance for any help, this has been driving me crazy for days now.