Using car radio output jacks for input
The RCA jacks on the rear of Sondpex RUSX903 Car Radio are intended for 'Line Out'.
You are indeed inputting music from your cellphone into the 'Line Out' jacks instead of the Aux Input jack which is on the front panel!
It works, though not as it should, because the 'Line Out' may be tapped after the preamplifier and the following stages can amplify the music you are inputting.
Even assuming no damage would be caused, it's not worth taking the risk when the appropriate input is available.