How to customize the extract for the Payments Check Template

On the check template for payments, the code passes the customer number in the supplier table. I need to customize is so that it passes the customer number from the supplier site table. I think it is the Create Printed Payment Instruction job that creates the xml that is used by the Format Payment Instructions to create check PDF. Where do I find the sql that is used to create the XML?  I have looked in the IBY_BUILD_INSTRUCTIONS_PUB_PKG package but I don't see any sql statements doing the extract so I am missing a piece of the puzzle.