'74 Duster Radio

The best solution would be to replace the dash speaker. The radio is mono, designed to power just one speaker. The green should be + to the speaker, the black should be ground.

The two speakers in the back will normally be wired green (left+) and green/black (left -) with the other side being grey (right+) and grey/black (right -). It's not a good idea to run two speakers from an old unit designed to run just one speaker, no matter how you wire them (series or parallel) you will most likely be changing the impedance load which is no good for the radio's amp. Adding the cassette player is the same deal.

If it was me, and I really wanted to use the two speakers in the back, I'd just replace the radio with one designed for two speakers.