This was discovered in my front door panel.
There's one on each side.

1991 XJ 4door Loredo. Power windows Power locks... WTF would this wire be?

 

Power mirrors an option back in '91?
Bit heavy for speakers.

 

Speaker connection for the upgarded radio option? Or power locks or windows if you don't have those.

 

I would think it is for power door locks being where the wire ends at .

 

OP states he has power windows and locks...right OP? Or was it a question?

 

Yes. I have PW and PL

I think it must be for a speaker... because there's an "x" cut into the clear plastic film that is just in front of a about a 4"square space in the metal door.
So if it's not for a speaker, I can't even guess...

 

I also have power mirrors.

 

Then I think you've answered your own question

 

It's for your power door locks. If you like I can take a pic of mine hooked up. I'm working on that door currently.

 

Yes. Answered my own question.

Power Door lock is working fine with this wire hanging loose as pictured.