Car audio cuts out when i have phone connected and volume is turned up to high.?

Some JBL speakers are 2-ohm. If that's the case with your speakers, then you may need to get an amplifier to run them, because the head unit is designed for 4-ohm speakers. If your head unit is staying powered up (ie, the display is still on) and it's just the sound cutting out, that means the internal amp is shutting off for protection. That's usually related to a speaker problem. On the other hand, if the deck is shutting off entirely (no display) then the problem is with the power and/or ground wiring.

I have been doing Car audio and electronics for 10yrs Professionally. Your problem is a power issue with the radio or a bad speaker. I had a customer with the same problem a few weeks where it would completely cut the radio off and shut down and restart after a few seconds when you hit a certain volume. and I had to run power directly to the ignition harness with fuses. and ran a seperate ground to the body of the car. and it fixed the issue.

ok first i gotta say to the so called installer charlie... your an idiot , the aux cable has nothing what so ever to do with the radio cutting off at a high volume so where ever you went to school ,go back an try passing the basic mecp exam cuz you suck, an to whoever goes to you for an install i hope they dont pay you. a bad aux cable will only make your speakers cut off on one side or the other at any volume wheather its high or low.any real installer with basic knoledge would know that. anyway now to answer you question with a real answer...you problem could be a few things ,usually when you turn the volume up an it cuts out its a bad ground in one of your speakers or your wires are shorting out cuz there touching eachother,this problem can be either at one of the speakers themself or at the head so go check all your connections an make sure they're all secure an tight. if that dont fix it than refer to the other guy under charlie that says he has over 10 years experiance cuz he has the right answer as well, its pointless for me to repeat his answer. also you said you have a jbl speaker in the back...i noticed you said speaker not speakerS so if you only have one than cherck an make sure the other speaker wire with no speaker attached is not shorting out or grounding out. im just saying this because you have me thinking you only have 3 speakers connected in you car an obviously you would normally have at least 4. good luck. .