Your converter or power supply should have two leads one plus (+) and one negative (-).
As you list the wires from the car stereo:
1. Black - This will have to connect to the negative (-) from your power supply (converter).
2. Yellow - This wire is the one that keeps constant power to your stereo for channel memory and clock settings. It will have to connect to the positive (+) on your converter.
3. Red - In a car this would connect essentially to the ignition switch so that turning the key on would turn the unit on. I think it usually provides the main power as well. It also has to connect to the converters positive (+).
(Yes, both the yellow and the red will connect to the same wire on the converter. Your stereo will lose radio presets and clock time when you turn the converter off.)
4. Blue - Remote lead. You don't need this lead. It goes hot +13.8volts when the stereo is turned on which would be used to remote turn on an amplifier or activate an antenna. (Some even have two, one for remote antennae which only goes hot when the radio is on and another for an amplifier remote turn on.)
Make sure about polarity +/- from the converter, you don't want to mix those up. You shouldn't likely be attaching anything to your barbeque.
Hope this helps.
Jim