This is how I would do it. For the Subtotal link, pass "ALL" for the Location Parameter and =Fields!.Customer.Value
for the Customer Parameter. For the Grand Total link, pass "ALL" for both Parameters. Then update your Sub-Report Query Where
clause:
Where
(Customer = @Customer or @Customer = 'ALL')
and (Location = @Location or @Location = 'ALL')
This will return all Customer/Location records when the respective Parameters are set to 'ALL'.
Let me know if you need any more detail.