Quote Originally Posted by dirtychinch View Post
Apologies for the cross post...I didn't realise that the forums were connected at all, and thought that this forum was more applicable for the issue. I requested that topic be removed on the other forum, before I posted in this one.
Evidently you haven't read what's in the link. The whole point about cross-posting etiquette that you'll read there is that the forums aren't linked and, by not applying the etiquette (which amounts to nothing more than common courtesy), you could end up having different people working on the same issue, possibly unnecessarily traversing the same ground and maybe even working at cross purposes.

As for:
I think the problem is that I've got 
    ActiveDocument.MailMerge.OpenDataSource _    Name:=SourceDoc, _
and it doesn't like how I've got SourceDoc as an input. This can't be avoided, due to the nature of how the program will be used.

I've got it working like this:

Sub MailMerge()
    Dim SourceDoc As String
'...
    With Application.FileDialog(msoFileDialogFilePicker)
        .AllowMultiSelect = False
        If .Show <> 0 Then
            SelectedItems(1) = SourceDoc
        Else
            Exit Sub
        End If
    End With
     
     'your connection string, also more easily separated with line breaks and arguments
    sConnection = "Provider=Microsoft.ACE.OLEDB.12.0;" & _
    "User ID=Admin;" & _
    "Data Source=""H:\Jonathon Hunter Hill\AirMaster\AirMaster V 0.9.xlsm"";" & _
    '...
    Name:="H:\Jonathon Hunter Hill\AirMaster\AirMaster V 0.9.xlsm", 
'...     
End Sub
But it's not acceptable for me to have the source in there, as I've said.
there's not much point in defining and selecting a source document, then not using it!

Your two references to:
"H:\Jonathon Hunter Hill\AirMaster\AirMaster V 0.9.xlsm"
should be replaced with:
SourceDoc