Depends on which system you have.  If you have a cassette player, the
cassette adapter is the cheapest and IMO the best solution.  If you,
like me, have the GPS system with a CD player, you have two options:

1)  You can purchase an adapter that plugs into your satellite/cd
changer port.  High quality.

2)  Purchase an FM modulator that splices directly into the FM antennae.
 You will have a toggle switch somewhere that you flip to shut off the
antennae and activate the modulator.  NOTE: this is NOT the same as an
FM transmitter.  There is no wireless transmission occurring. It is
wired directly into the antennae.  Medium quality.  There is no static,
but the highest quality you will get is that of an FM station, giving up
highs and lows.

I didn't want to give up my satellite radio with option 1.  Option 2 was
my only choice ... and it was the cheaper of the two.

You can have either option installed at your local car audio shop, like
Car Toys.

Good luck.
Craig C.