Hello,
Section #7 on https://forum.multitheftauto.com/topic/118473-knowledgebase-modding-faq-and-help-thread/ explains how to unwrap models for the 2nd UV channel. The 2nd UV channel is used for environment maps (reflections). For your case, I'd imagine working with UVW Map or UVW Unwrap modifier and enabling Cyllinder projection mode would get you the results you're looking for.
The envmap texture can be any texture, but is usually the vehicleenvmap image from vehicles.txd found in GTA SA installlation\models\generic\