My understanding is due to the extreme heat and humidity changes in a car prolonged storage of water is not recommended. If you are going to do it the US Coast Guard does have a safety standard set for survival water in life rafts and I would use something that meets those requirements.
Emergency Water Pack -3 Day Survival Rations (6x4.2oz. Pouches) 5 Year Shelf Life USCG Approved https://www.amazon.com/dp/B0075ZH3SU/ref=cm_sw_r_cp_api_glt_fabc_G1JYC14BWKTZXC58H3WZ