I had that plastic on plastic squeak in my dash before. Sounds dumb, but try wiggling the a/c line where it goes through the dash. Side to side and a little back and forth. Mine stopped for a few months after that. It eventually squeaked again last spring so I gave it another wiggle. No squeak since.

 

...dead...

 

I have an 09 JK Rubicon. I have heard this noise for 2 1/2 years and it sounds like something fell down behind the dash and is rattling around in the dash between radio and glove box. Well after 2 1/2 years we were in Moab, UT riding the slick rock a couple of weeks ago and my wife reaches up to turn the a/c vent in center of dash on her side right between the radio and the glove box and I noticed that when she touched it the rattling stop. WhaLa!!! Now I just wedged a small piece of cardboard from a business card in there and no more rattling. I still can't believe I went that long without finding the source.

Doug Hines

 

I have had a rattle in the dash since the Jeep was new (08 JKUR). Have asked the dealer to look into it, they have never fixed it and I don't think they have tried. Several years ago I read a thread where the problem was the AC tubing going into the firewall on his Jeep.