training help

hi… so my two year old rottie is having an issue with lunging at cars - i’ve been training him from the beginning (he knows basic commands and heel things like that, also good engagement while walking with me) and don’t know where i went wrong because this never was an issue like it is now - makes me feel as if i’ve failed him and he’s the first one i’ve trained by myself in my family so do any of you guys have tips for this issue so i can fix it please and thank you. it’s especially bad when a car is coming from behind - no barking or anything just lunging.