You have to determine whether the outs from the radio have a common ground. The chances are good that they do, although automobile equipment does sometimes use a bridged configuration to get more volume from the low battery voltage.
You need a multimeter with a continuity test feature, a setting that makes a sound when the probes are touched together. This is usually the lowest resistance range.
Touch one probe to one of the output wires on one channel. Try the wires on the other channel with the other probe. If you get a sound from one you have identified the grounds on either channel. If you get no sound try the same test using the other wire on the first channel.
If you have discovered that the outputs have a common ground then you can wire them both to the sleeve (shaft) connection of the jack socket. Tip is left signal, ring is right channel signal.
You said your helmet has an amp. This will have a high input impedance so resistors are not required. All you really need is to connect everything up and set the radio volume to a level which gives you a good range of adjustment on the helmet amp without excess hiss (too low) or distortion (too high). Start with both volumes set low so you don't damage either your ears or the equipment.
If you don't discover a common ground, things are a bit more complicated and a lot more long-winded to explain, you'd be better to ask for some help from someone knowledgeable who can look at the equipment. Don't proceed any further without some more input, because it's entirely possible to destroy the radio by connecting the outputs together if they are not suited to the arrangement.
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