I have 10 Workbooks that all have identical Modules and VB code functions.

I want to centralize that code in one place so that fixes don't have to be
made 10 times. I created an Add-in with a Module containing all of the
common code. The problem now is that this common code refences the worksheet
cells (which don't exist in the Addin) as ThisWorkbook. I want it to
reference what ever workbook was opened. How do I do this?

So for example the code in the Addin Module1 has:

Set wsh = ThisWorkbook.Sheets("DW")
ThisWorkbook.Sheets("DW").Activate
'Assumes data starts in column D
Set rRng = wsh.Range("D11", "AY11")

On Error GoTo Err_Handle

sContractCode = Worksheets("DW").Range("D5").Value
dStartDate = Worksheets("DW").Range("D7").Value

etc.,etc...