The good news: every visitor that your vendors send to your website can be tracked through Google Analytics. The bad news: it doesn't happen automatically.
In order to see the true number of visitors from vendors that send your site a substantial amount of mobile traffic – like Cars.com, AutoTrader, and Pandora – all the links pointing to your site need to be tagged to play nice with Google Analytics.
As smart as Google is, it can't detect the source of a visitor arriving from a mobile app – and neither can any other analytics tool.
Instead, Google Analytics will label it as a "(direct)" visit in its reports, which means it won't show up when you look at Cars.com and AutoTrader referral traffic. Pandora traffic is an even bigger issue, since the traffic it sends is almost entirely mobile-focused – typically at or above 90% – and a huge chunk of it will come through the mobile app. So unless it's tagged, you're unable to see the impact of that traffic.