Wiring diagram for 1967 Plymouth AM/FM radio

The radio was likely designed to drive a single 8 ohm speaker. Those are hard to find in typical car audio stores. Most car speakers these days are between 2 and 4 ohms.
The correct way to wire the radio to a pair of 4 ohm speakers would be in series.
Green wire to positive terminal of speaker 1.
Negative terminal of speaker 1 to positive terminal of speaker 2.
Black wire to negative terminal of speaker 2.
This will result in the speakers being wired in proper polarity, and will present an 8 ohm load to the radio.
The power delivered from the radio will be split by 50% going to each speaker. Those radios were good for about 6 watts total.

If you attempt to connect a pair of 4 ohm speakers to it wired in parallel, that would result in a 2 ohm load, and you will likely damage the output circuit of the radio.

I would not suggest connecting an amplifier to this radio. Technically it is possible, but you need to check and understand a few fairly technical things first to avoid any damage.