Have you checked the cable as well?
1/8" to stereo RCA cables are notoriously poor quality. You will want to connect that cable DIRECTLY to a source which has a 1/8" stereo audio output and ensure that audio plays properly over both left and right channels as expected. I recently did an installation where audio sounded fine from multiple sources until I got to their 'aux' input. This was (sure enough) a 1/8" to stereo adapter. It was an older adapter, and I had a spare with me. Swapped it out and audio suddenly sounded perfect!
Other checks, as listed, are to use an input that you already know is 100% perfect. Like your CD or DVD input. Start up your CD player, play some music. Make sure it sounds perfect. Then unplug your CD player (fine to do while it is running) and plug in your 1/8" stereo to RCA connection and plug it into your phone. If you don't hear audio from both channels, then odds are good it is your cable. If you hear audio from one side only, then unplug the other side and see if you hear a change. The swap the good side for the bad side and see if audio flips which speaker it is coming out of. Indicates a bad cable.
If audio plays fine out of both speakers using your CD input...
Plug your CD player into the TAPE IN connection.
Then switch to your Tape input on the receiver. Does audio sound good and playback properly from both speakers? If so, then you know the Tape Input is working properly on the receiver.
Put the CD player connection back where it belongs.
I would put my bets on the cable being the issue.