Your passenger hears what's in front just like you do. So rears aren't a necessity, but 6x9s certainly add more midbass and sound up to about 65-70mph. Horns aren't going to deliver clarity, just loud. I'd suggest Soundz Iron 65 uppers and lowers with their 4channel amp and HD-DSP for very clean, loud, and great midbass response 80mph+. Then look to add rear 6x9s cut into stock lids, still using the same amplifier. 800 true watts.