Reduce interference over 3.5 mm to cassette tape adapter
Looks like this will stay open, so:

It sounds like you are getting RF interference from your cell phone.  There's a few ways to go at it.


Simplest and cleanest is to get a split ferrite bead, make a few loops in your cable, and clip the bead on it. The bead will block RF traveling down the cable.
Connected that foil around the cable to the chassis of the car. Without the ground (car chassis) connection, the RF hits the foil and the foil re-radiates it.  With the ground, the RF hits the foil, follows the wire down to the chassis, and goes away from your electronics.
Ground the shielding in the cable.  Your cable is made of a couple of shielded wires, like headphone cables.  You could carefully remove a spot of insulation from each of the pair, and run a run from the shield under the insulation to ground (car chassis.)


Number 2 is ugly.  

Number 3 is ugly and if you aren't good with stripping wires and soldering you could mess up the cable beyond your ability to repair.

Number 1 is the cleanest and probably best way to go.

You might find an old ferrite bead on an old wall wart cable that you could scavenge. 



If you don't have a ferrite bead (or to kill time while waiting for one to be delivered) you could try this:

Wrap the cable around some soft iron (like a nail) just an experiment.  It may well reduce the interference enough that you don't need anything fancier.

If wrapping it around a nail doesn't help, that doesn't mean a ferrite bead wouldn't help.  It just means nails can't do the job of a ferrite.