The van I got came with a Sony radio uninstalled. There are two harnesses hanging out as well as the antenna cable and speaker wire. Can you tell from this crap show of a rat nest which one(s) are needed to hook into the radio?

 

You probably need it all. You will have two for radio power and a pair for each speaker. You didn't mention how many speakers there are.If you have no access to a manual I would go to each speaker and note the color sequence and polarity for each speaker. You should then be able to match them up at the radio end. The remaining two which should reside on their own connector will be radio power. You can easily determine polarity with a meter. The antenna is straight forward.

 

It only has one speaker (center of dash) and those wires are coming off of the speaker itself. With that said, any idea which wire(s) are needed to power up the radio?

 

I am looking at my 87 manual, test with a volt meter as I am not 100% on this. Look for a light green/yellow for radio memory which is hot all the time and a yellow/black for switched radio power. Solid black would be ground.