What was found
FLT 603 - DFW-MEX - AC CROSS TIE LIGHT TURN OFF, THERE WAS NOT A CROSS FEED AND APU O/S. CIRCUIT BREAKER OF GCU, GENERATION AND INTERCHANGE WAS RESET AND AIRCRAFT WAS DISPATCHED BY MEL WITH APU O/S. DURING TRANSIT CHECK IN MEX IT WAS REPLACED THE GCU OF THE APU, HOWEVER CROSSFEED WAS NOT POSSIBLE. REPORT DEFERRED BY MEL 24-3 CAT 'C'. LATER WAS REPLACED THE BCU IAW MM 24-20-05. AT RON MEX 22/23 SEVERAL TESTS WERE PERFORMED IAW 24-20-08 AND 24-20-07. (M)