A B737-300 FLC TAXIES THE WRONG RTE ENRTE TO DEP RWY AT ORD; IL.

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OUR FLT WAS CLRED FROM OUR GATE AT ORD TO RWY 32L AT T-10 INTXN VIA TXWYS B; M2; M; T. AS SOON AS OUR CLRNC TO TAXI WAS ISSUED; THE GND CTLR IMMEDIATELY BEGAN ISSUING CLRNCS TO OTHER ACFT. I WAS UNABLE TO READ BACK THE CLRNC RTE TO RWY 32L AT T-10. THE CAPT TAXIED THE AIRPLANE TO EXIT THE RAMP. I REPEATED THE TAXI RTE TO HIM; AND THEN I BEGAN TO PERFORM MY NORMAL BEFORE TKOF CHKS. DURING THIS TIME I WAS 'HEADS DOWN.' OUR FLT ALSO HAD THE ADDITIONAL PROB OF BEING AT A WT WHICH REQUIRED US TO CONTACT DISPATCH. I WAS TRYING TO CONTACT DISPATCH VIA ACARS; THEN BY VHF COM. I LOOKED UP TO SEE WHERE WE WERE. THE CAPT HAD TURNED ONTO TXWY T FROM TXWY A7; AS OPPOSED TO TXWY B. I IMMEDIATELY ADVISED THE CAPT THAT WE HAD TAKEN A WRONG TURN. THE CAPT IMMEDIATELY STOPPED AND WE CONTACTED GND CTL TO ADVISE THEM OF OUR MISTAKE. GND ISSUED NEW TAXI INSTRUCTIONS FOR US AND WE CONTINUED TO RWY 32L AT T-10 VIA TXWY T. I BELIEVE THIS ERROR WAS CAUSED BY MULTIPLE FACTORS; INCLUDING: FREQ CONGESTION; INABILITY TO READ BACK A CLRNC; BEING ASSIGNED A TAXI ROUTING THAT WAS OTHER THAN THE EXPECTED ROUTING; HAVING A SHORT-TAXI TO THE RWY; WHICH NECESSITATED ME TO BE 'HEADS DOWN' SOON AFTER WE RECEIVED OUR TAXI CLRNC; AND HAVING TO CONTINUE TO BE 'HEADS DOWN' WHILE I TRIED TO GET IN TOUCH WITH COMPANY DISPATCH. THIS PROB COULD HAVE BEEN AVOIDED BY ENSURING THE CAPT (25+ YRS EXPERIENCE) UNDERSTOOD THE TAXI RTE; AS OPPOSED TO ASSUMING HE KNEW IT DUE TO HIS VAST EXPERIENCE AT ORD.