Help me read this car stereo wiring diagram

Factual Questions
1

http://carstereohelp.net/wireharness_Nissan3.htm

Ok, I just bought an old Nissan factory car stereo off eBay. It’s supposed to fit a Nissan Stanza '90-'92 and it almost does. But not quite.

All the wiring harnesses fit in perfectly. The only problem is that slot that says “SUB” and “MAIN” in the wiring diagram. My old car stereo only has a single input here. The new car stereo has a single plug with a dual jack.

I’m an idiot when it comes to this. All I know is that when I unplug this, I get no power on my old stereo, so I assume it’s some kind of power source. What do I have to do to make the single plug input of the old stereo fit the dual plug input of the new stereo? What does “SUB” and “MAIN” mean in this context? I’m comfortable soldering, stripping wires, and all that if it makes any difference. But before I start stripping and soldering, I want to know what I’m doing.

Thanks

2

The link in your OP indicates that receptacle with Sub and Main is for the antenna. Is it possible that your original vehicle has a fixed antenna, and the one you got from eBay is designed to control a power antenna, as in key off or radio off-antenna down, key on and radio on-antenna up?

3

I just figured it out. Nissans have some sort of proprietary antenna plugged called a “Diversified Antenna.” For some reason, though, my Nissan (which I bought used) has a standard antenna plug.

4

For better FM reception many cars have two FM antennas. On Volvos, the second (diversity or sub antenna) is either located in the rear bumper, or in one of the fixed glass windows.
In any event, the radio shoud work, and receive FM if you plug your antenna cable into the main socket.