To turn 12 Volt DC to 5 Volt DC, you simply put two resistors in series. This is called a voltage divider. The voltage drop on the first resistor is 7 Volts, and on the second it is 5 Volts.

The formula is E (Voltage) = I (Current) * R (Resistance)

12 Volts divided by 1/2 Amp( 500mV) = 24 Ohms resistance.

5 Volts divided by 1/2 Amp = 10 Ohms. 7 Volts divided by 1/2 Amp = 14 Ohm. The total of 10+14 = 24 Ohms resistance.

.

o---------
|
R1=14 Ohms
|
12 Volts ----------------o
|
R2=10 Ohms 5 Volts
|
o------------------------o

However, if the FM transmitter needs 500mA and the Shuffle needs 500mA, then something might be starved for power. The is a good chance that it will work. You would have to try it.

Ignore the above post about DC-DC power inverters. That would be better suited to stepping up voltage from 5 Volts to 12 Volts--much more difficult; and it would be a converter not an inverter. An inverter is for making AC from DC.