Using car radio output jacks for input

On the backside of my Sondpex RUSX903 car radio I found two RCA jacks: a red and a white one. With an adapter cable I connected these to my cellphone headphone jack. So far so good, I noticed two odd things though:

• The volume control of the car radio does not affect the volume of the music coming from my cellphone. I gets pretty loud though when I increase the volume on the cellphone.

• The music from the cellphone plays independently of the selected input. If the selected input has some audio as well (e. g. the radio,) I can hear them both mixed together.

Recently a car electrician saw my setup and told me the RCA jacks I’m using for input are in fact output jacks.

Questions:

1. Why is this even working? Why is it even possible to use the output jacks for input?

2. Is it safe,or should I expect to run into problems or even damage something?

EDIT:

The Aux Input Jack on the Front Panel of the Car Radio is broken.