my "68 has what i would guess to be an AWG 10 wire - my wire tables are not available so i can not tell you if this is adequate or not. you can check adequacy of alternator by measuring 13.5 +/- 1.0 volts across battery terminals with the added stereo off and then on. if alt can't handle stereo voltage will dip below specified voltage which would indicate overload or wiring for stereo could be causing problem.