That "guitar input" is actually what connects the radio to the antennae. What you need to do is test the wires to find which one's power, then either find the ground wire or run a new one.
To test, get a digital multimeter and hook the positive wire on the multimeter up to a wire, and the - to a metal surface on the car (ground). Check each wire with the + to see which one's the power. Of course, battery has to be on to do this.
Once you've found the power wire, turn off the car and connect (tie, crimp, or solder) that wire with the (usually red) power wire on the car. You'll also need to find the ignition wire (usually yellow) and connect that to the yellow wire on the radio so that the radio turns on & off with the car's ignition.
After that, screw around with the other wires until you get sound coming out of the right speakers.
