If the radio doesn't have separate front and rear outputs, you'll have to run the wires in series. This method will result in half the load being put on the head unit....in other words, if you're connecting two 4 ohm speakers, the radio will only see 8 ohms of load. Not a bad thing. This is accomplished by wiring the radio right positive output wire to the front right speaker positive terminal....then run a wire from the front right speaker negative terminal to the right rear speaker positive terminal...last, run a wire from the right rear speaker negative terminal back to the radios right negative output wire. Repeat the same for the left side using the left output wires from the radio and you'll have 4 speakers with sound. You'll only be able to control balance with this method and cannot fade front to rear, but it's the only safe method of wiring 4 speakers to a 2 speaker output radio. If you simply wire the right side positives and negatives to just the two right side wires (+/-), you'll be placing a 2 ohm load on that stereo unit and I can assure you it won't be long before the radios onboard amp cooks from overexertion.
Radio Rt + to Front Spkr + terminal
Front Spkr - terminal to Rear Spkr + terminal
Rear Spkr - terminal to Radio Rt -