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Most parts houses won't carry SXL/GXL wire, because it's more costly and many techs don't know the difference. GPT or PVC thermoplastic insulated wire may be all you'll find. But, you can order SXL/GXL: Del City or Terminal Supply Company, etc.
...Flickering head lights and dash lights....
...Connect your voltmeter between the alternator output post and ground....
Alternator voltage is fluctuating between 13 and 15.3 under load. I tired the reg wire to the charging post. Still the same thing. . Some reason I'm thinking it's a ground issue idk