Making Customer ID visible in Invoice and Packing Slip

Hi,
How do I make the Customer ID visible in addition to the Customer name and Address in Invoice and Packing Slip.

Many thanks

What do you mean by 'customer id' ?

Each customer get assigned a unique ref, when they sign-up. This is visible in Admin > Customers > Customers and its the first column "ID#" - This doesn't normally appear in either the Invoice or Packing Slip, would it be possible to make this field visible along with the Customer Name and Address.

Thanks

There's this:

http://www.zen-cart.com/forum/showthread.php?t=180487

and maybe this:

http://www.zen-cart.com/forum/showthread.php?t=860433

Thank you for the links, I'll try the suggestion, even though its not for the emails, I would like the ID made visible when manually printing by going to Admin > Customers > Orders and then clicking on either "Invoice" or "Packing Slip" buttons. ie: is there an invoice.php or a packing_slip.php needs modifying and if so, the filed name which I should include and approximate line number as to where it should go.

The 2nd link doesn't seem to work.