iPhone 6s plus unexpected shutdown and not getting turn on

hello guys,

My phone died the other day unexpectedly, although it was charged like 60 percent.

First i tried force restarting it. but it didnt help. then i plugged it to be charged. Nothing happens as i plugged it to power. however, after like six hours it turned on by itself. next day it again died and again after some hours it just turned back on. this has happened to me like four times by now.

i have tried force restarting it. i have reset the phone using the phone when phone was on. i have recovered it using itunes when the device was working. but non of this has helped.

i should say i has not been damaged physically or has not been wetted.

When for the first time it turned off, my cars AUX jack was plugged to the device's headphone jack.

Guys i dont have access to apple stores or their repair centers. so please help me what could have caused this problem.

thank you