Start with the battery and check voltage. If that is good, use a clamp meter, everyone should have a good one, and measure the current from the battery. If everything was off then it should be close to zero. There are things like the stereo that uses a bit. But the reading should be less than a few amps. If all is good them check the voltage at the radio. If it is less that the voltage at the battery then there is a voltage drop. These are caused by poor connections or corroded wire, poor connection in an in-line fuse holder etc. Check all the way back to the fuse panel. Verify that all the connections at the panel are tight.
