music volume is too low using aux input in car

Do you have the phone volume max'd, or just the car's volume? In my Honda Civic, I put the phone's volume in the music app to max (and no uoper limit set in music settings) and then I can adjust it just fine from then on with the car's volume.

First check all the easy stuff. Phone volume, EQ, cables, etc. Some car stereos have volume settings for the AUX in a menu. After all of that, if you find that you still need more volume, you may want to look into a preamp. There's lots of them out there. Check out the offerings from Boostaroo, FiiO, or Sound Shocker.