Forgive me for being a relative Tesla noob, but how does Slacker and TuneIn work? Do you have to rely on the data connection/signal within the car? Or do you tie it in with your phone? I wouldn't mind being able to forgo the high fidelity sound system if I could listen to decent music on Slacker whenever I want. Is there a subscription service to Slacker?
It relies on the data connection from the vehicle. You'll get a 4 year subscription to data when you purchase the vehicle (at least that's how it currently works). Slacker is also included at a custom Tesla subscription level for that period. You can upgrade it if you desire, some have done so. TuneIn is free but does rely on the data connection.
Some owners will just turn their phone into a wifi hotspot and let the car connect to that; that'll likely be the way I go once my vehicle plan expires.