On the back of the radio...I've googled enough to find out that it can be called a PCI connector. I have no idea what that means or what it does.

I have a 2000 Cherokee sport, no steering wheel controls or anything, and the base radio with cassette player.

 

I remember seeing a similar connection on other brands and it was for communications between the head unit and a cd changer.

Ok, looking thru my 97 FSM i see a single wire connector that looks like yours. It says "Ground". I dont remember seeing one when i replaced the factory head unit, but my memory has been slowing turning to crap lately.

 

Three things argue for it being a ground:

1. fb's finding in the FSM
2. The fact that it's a solid black wire (standard for a ground)
3. It's a direct connection to the case of the radio, which must be grounded.

 

There's definitely a ground ring terminal on the back. it goes in the hole that you can see on the back of the radio, fastened down with a 10mm fastener.

The truck does have an aftermarket CD changer. It was installed near the spare in the rear. I bet that's what the extra 'splitter' connector is too. If you look closely, the light grey connector has an interrupt in it, and at least two of the circuits that are installed with a butt connector are the power and ignition feeds, to turn on the cd changer likely.

I'm going to put the radio back together with that extra wire secured with a tie strap but not connected to anything and see what happens.