A B737-400 FO AND PIC DISAGREE ON THE RELEASE PROVISIONS ASSOCIATED WITH AN MEL PROC WITH AN UNRELIABLE L PACK IN MMGL; FO.

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AFTER PARKING AT THE GATE AT GDL WE FINISHED THE SHUTDOWN CHKLIST AND THE CAPT LEFT TO GET THE NEW PAPERWORK. SHORTLY THEREAFTER A L WING-BODY OVERHEAT LIGHT ILLUMINATED. I RAN THE QRH CHKLIST WHICH REQUIRED THE APU TO BE SHUT DOWN. NO JETWAY ELECTRICAL PWR WAS AVAILABLE. WE THEREFORE HAD NO AC PWR OR COM CAPABILITY ON THE ACFT. WHEN THE CAPT RETURNED I EXPLAINED WHAT HAD HAPPENED AND WE RECHKED THE QRH PROC TO INSURE COMPLIANCE. THE CAPT THEN WENT TO OPS TO CONTACT MAINT CTL WHILE I TRIED TO ARRANGE FOR A PORTABLE GPU. THE CAPT RETURNED AND INDICATED THAT MAINT WAS GOING TO DEFER THE INDICATING SYS. I TOLD THE CAPT THAT I WAS NOT CONVINCED IT WAS THE INDICATING SYS OR THAT WE WERE MEL'ING THE CORRECT PROB. HE RETURNED TO OPS AS WE STILL HAD NO PWR ON THE ACFT; AND RETURNED LATER. HE SAID MAINT WAS AWARE OF THE PROB AND THAT WE WOULD RETURN TO LAX WITHOUT USING THE L PACK OR APU TO PRECLUDE ANY FURTHER PROB. I SUGGESTED THAT THOSE ITEMS WOULD ALSO HAVE TO BE MEL'ED AND HE RESPONDED THAT THEY DID NOT AND THAT THE MEL WE WERE USING WAS CORRECT. WE RECEIVED AN AMENDED RELEASE WITH THE MEL AND A NEW FLT PLAN FOR FL240 FOR THE SINGLE PACK OP. AS WE WERE NOT USING ANY BLEED AIR FROM THE AFFECTED SIDE I WAS CONVINCED THAT THE CONFIGN WAS SAFE AND COMPLIED WITH THE CAPT'S INSTRUCTIONS.