SQL - How can I get the replacement part description on a query

I have a part_code and a part_desc (Description) and a rep_part(Replacement).  There is no specific field in the database for a replacement part description.  How could I get that?  It is another part of the part master.

Example

Partmaster for Part_code 1234
part_code = 1234 Part_Desc = Numbers  Rep_part = ABCD

Partmaster for Part_Code ABCD
part_code = ABCD Part_Desc = Letters Rep_part NULL

In this example the rep_part ABCD in the part master would have a description of Letters

How can I get those on this query?

SELECT DISTINCT 
                      ediorderrors.order_numb, CAST(ediorderrors.item_no AS INT) AS line_numb, ediorderrors.error_desc, CAST(ediord.cust_code AS int) AS cust_code, ediord.loaddt, 
                      ediord.part_code AS UPC_CODE, partmstr.part_code AS PART_CODE, partmstr.part_desc AS DESCRIPTION, partmstr.uom AS UOM, ediord.shipto_code, 
                      ordhead.shipname, ediord.qty, partmstr.rep_part,ordhead.order_numb,rep2
FROM         ordhead INNER JOIN
                      ediord ON ordhead.cust_po = ediord.cust_po RIGHT OUTER JOIN
                      ediorderrors ON ediord.item_no = ediorderrors.item_no AND ediord.cust_po = ediorderrors.order_numb LEFT OUTER JOIN
                      ediorditem ON ediorditem.item_no = ediord.item_no AND ediorditem.cust_po = ediord.cust_po LEFT OUTER JOIN
                      partmstr ON partmstr.upc_code = ediord.part_code
WHERE     (ediorderrors.cust_code IS NOT NULL) AND (ediord.cust_code <> 'ZZZZZ') AND (ediord.cust_code <> 'PETLAN') AND (ediord.loaddt BETWEEN '09/11/14' AND '09/12/14') AND 
                      (ediord.cust_code <> '068775311CUSPRD') AND error_desc <> 'This is an invalid part ... Please Re-enter.'

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