Wiring issues while trying to install a stereo
 

I am trying to install a pioneer stereo in my wife's 2000 chevy blazer and the stick stereo had 2 plugs in the back with wiring. 1 plug had a bunch of wires and I'm assuming that is for the stereo because it has speaker wires. The other only has about 5 or 6 wires to it and that is the one I am not sure what it goes to or how critical it is.
https://cimg1.ibsrv.net/gimg/blazerf...2c9d87268c.jpg
This is the one with the speaker wires
https://cimg5.ibsrv.net/gimg/blazerf...6ed8e67673.jpg
This is the plug i am not sure about
https://cimg3.ibsrv.net/gimg/blazerf...1e2e42133f.jpg
These are the wires going to the smaller plug

That one looks like it might go to the cassette deck.

The stock stereo had a cd player would it still plug into the cd player if it is for a tape deck? I was told possibility of it being the instument plug?

You wouldn't use it at all with an aftermarket stereo (if that's what it really is).

That's why I am trying to figure out exactly what it is so I know if it is a crucial component or just something extra.

Here's the wiring diagram for it.

The grey/black plug has all the wires you need for an aftermarket radio. I'd suggest you get an adaptor plug for it rather than cutting the plug off and attaching directly to the factory wires. They're really cheap and make the job a ton easier.

The black plug is for a remote cassette player that would be located in the centre console under the heater controls. You can just leave this tucked inside the dash. It does not need to be plugged in at all. The harness may be there even if the truck did not have the factory remote cassette player option installed.