uni-body and a frame are the same general principle, both connect to each other with bare metal, your body doesn't always connect to the frame with bare metal, its normally painted.
Resistance increases with heat but the heating of the wire is based on the amperage flow through the wire to begin with. Heat is expressed in watts. Heat created is defined as voltage drop across the load times the curent flowing throught the load. Voltage drop is defined by current times resistance. Since there is only voltage drop when current is flowing and the voltage drop is proportional to both the current and the resistance, you need to minimize one or the other to minimize voltage loss. Since you need the current to make power in the amp, it is only practical to minimize resistance. This is done with either a larger conductor or a conductor with higher conductivity. The other thing that has to be considered is that since the power loss is proportional to the current, in the case of a low current consumption system, the power loss will still be negligible even with smaller, comaparatively high resistance wire. Negligible power loss also means negligible heat in the wire.
Don't forget that you can lose voltage and thus power on both the positive wire and the negative (ground) side as well. If you are using a chassis gournd, you don't have to worry about heat (the metal of the car makes a very good heatsink) but you will still lose power because of the relatively high resistance of the metal in the car. Steel is 10x more resistive than an equivalent cross section of copper. Therefore to have less resistance than a run of copper wire, the conductive path through the car body must have at least 10x the cross sectional area of the wire. When you consider that most unibody cars are only tack welded and glued together, there isn't a large continuous conductive path back to ground and using wire rather than the chassis becomes a much better choice for ground connection. A unibody and frame are similar what they achieve structurally in the car but the similarity does not extend to the electrical side of things
