How to calculate DC power consumption of a device
Watts = Volts x Amps

So, if you measure the amps flowing through the circuit, then multiply it by the voltage across the circuit, you will have the number of watts the system is using.

Switch your volt meter to a suitable voltage range and measure the voltage across the battery (should be somewhere around 12V).

Switch your volt meter to measure amps (if it can - if not get one that will) and connect it in between the battery and the stereo:

  ----(meter)--------
  |                 |
  +                 |
Battery           Stereo
  -                 |
  |                 |
  -------------------


Multiply the two readings, and voila!