What was found
REJECTED TAKEOFF AT 30 KTS DUE TO LOSS OF ALL ELECT POWER EXCEPT EMERGENCY POWER. MULTIPLE PROBLEMS RELATED TO RAIN DRIPPING INTO AVIONICS BAY. SEE ALSO WRI 25-70-1318, AFTER REJECTED TAKEOFF TAXIED BACK TO BLOCKS AND INSPECTED AVIONICS BAY. FOUND SIGNIFICANT AMOUNT WATER IN AVIONICS BAY. NR 2 IDG PREVIOUSLY ON MEL PRIOR TO THIS EVENT. CHECKED DRAIN PANS, FLOOR SEALANT, DRAIN LINES, ALL CLEAR. ACFT LOADED IN HEAVY RAIN. ACFT DRAINED OUT. REMOVED AND REPLACED NR 2 GCU AND ALL ASSOCIATED SYSTEMS OPERATE NORMAL ON ACFT A300-600 MM 24-41-11-4 AND 24-41-00-0.