How to use relative path in WiX toolset

[sansa20]

I am trying to create an msi file by including several binaries into it. The path of the binaries are all relative, and I have tried using the following code to achieve this.

<Component Id="OpenTap.Package.dll" Guid="PUT-GUID-HERE">
<File KeyPath="yes" Source="$(var.SourceDir)\OpenTap.Package.dll" />   
<?Component>

I have created a config.wxi file where I have included the definition of var.SourceDir, something like the code below.

<?xml version="1.0" encoding="utf-8"?>
<Include>
	<?define SourceDir = "D:\Workspace\blah\blah"?>
</Include>

But my SourceDir definition right now is hardcoded so it wont work if I try building on other PCs or the pipeline machine... So is there any way I can get my sourceDir to be a relative path as well so that the code works on all PCs? I have tried replacing the SourceDir value with a path like "......\blah", and it works just for some of the binaries. I am not sure if this approach works for the few binaries just by fluke. Or is this the right way to go about with it, and the other binaries that are not working is the issue?