So I got a new HTC Incredible Droid a week or 2 ago and it get Pandora without commercials at no extra cost.  So I wanted to put an Aux Head Phone Jack in the dash so I could plug into it and no longer have to listen to commercials in the XJ!!!  This is how I did it.  Notice, I have an after-market stereo with a system in my XJ so this install will vary based on what plugs are open on the back of your radio.  You should pull your radio before buying any plugs and see what you need.  Also this is a '99 XJ so if you are before 97 your trim will be different.

The only open plug on the back of my radio is for a 6 disk CD changer.  I had to get a plug that fit that, and then Y into a Dual RCA cable.  You might have RCA inputs available to be used on the back of your radio, so look first.

These are the items that I used:

1) (1) Dual RCA  Audio Input cable (male) to 3.5mm Stereo Cable (male); $9 at Radio Shack (should have bought on online from Ebay or something, could have gotten it cheaper.

2) (1) Apline Ai-Net RCA Aux Auxiliary Input Adapter; $5.49 (shipping included) on Ebay

3) (1) Stereo 3.5mm Panel-Mount Audio Cable; Found HERE, 10.95 + $10 shipping (OUCH), worth it to me though, was the only thing I could find that looked as good as I wanted that I did not have to wire the other end.

4) (1) Female to Female 3.5mm Coupler, $3.99 at Radio Shack ( I should not have had to buy this but I screwed up when ordering the cables and ended up with 2 male ends, remember measure twice and cut once...)

  Pictures of Said items with coresponding numbers:


Item #4:





So after getting all of your cables purchased and ready to go, now is install time.  First thing is to take off the driver's side front tire.  

Just kiding, first thing is to take off the beauty ring on the dash (don't know what to really call it).  It is easiest I have found to start at the bottom and pull that off first and then the top will follow.

Before:



After:



After that you remove your stereo.  Mine was only heald in there buy 2 screws.  If you have a stock stereo you might have to have a special tool (I don't know about XJ's I have always had a after market stereo in both of mine).

This is what the back of mine looks like:



The plug on the far right is what I will be pluging into.  This is a pic of the end of plug #2 from the top of the write-up:



After plugging that in you attach the RCA end to plug # 1.  Once you have that done technically the install is done.  If you want to stop there you will still be able to use it, you will just have to leave a wire hang from the dash.  Once I got to this stage I plugged it in to my phone to make sure that it worked.  That when if when I got to the end, and it didn't work, I would know that THESE cables were working properly.

Everything checked out good.

The next thing that I did was drill a hole in the dash.  I didn't want to remove anymore of the dash so I drilled it right there.  If you are going to put the Jack in the same spot that I did, don't be like me, remove the dash.  I ended up having to remove it anyways to get it to plug in, and I could have made it look better by removing it.

Here is the pic after the hole was drilled.  Yes, the bit walked on me a little so it is not lined up straight...



So I shoved the threaded end of plug # 3 in the drilled hole.

Here is a pic of the threaded end:





I wanted this plug so that I could screw it to the dash without it looking hideous.  I think that it looks good, you can say what you want.

After I put it in the hole I realised that the plastic was to thick in this spot.  SO, I had to take the dash off which was only 4-5 screws (should have done this to start).  I will say that getting the headlight switch through it's hole was not the easiest.  I just pushed real hard to get it off and on, forcing it through the hole.  I don't think that this is the right way to do it (I thought that I was going to break it) but it worked.

So since the plastic was to thick, I took the dash off and drilled the hole bigger from the back side but did not drill all the way through so that the hole on the front was still as small as I needed it.  BE CAREFULL.  The first attempt at this almost sucked the bit all the way through the hole and would have made it to big to thread the nut (or what ever you want to call it) back on to hold it on the dash.  The best way I found to do it was to put the trim on the floor with the back facing up and put your hand both on the ground and holding the end of the drill.  That way you can control how deep the drill goes.  Go slow I almost screwed myself on 2 seperate occasions...

Here is what the hole looked like from the back when I was done:

Then put the Plug #3 end in and it fits like it is supposed to:

Without the threaded nut on the end (so you can see how far out it stuck)



With the nut threaded, came out perfect if you ask me:



Then if I would have bought the right cables I would have been able to plug the 3.5mm male end of Plug #3 or plug #1 into the female end of the other.  But I screwed up and bought to males so plug #4 combines them and boom some magic happens and we are in business.

Here is the final pic of everything back installed:



The plug is up on the left side of the radio.

I have bought a 3.5mm plug with 2 male ends that is on a reel as well, it is just not here yet.  It will be velcrowed on the dash up by the windshield where I cannot see it.  I have a phone holder that suction cups to the dash as well.  So I can put my phone in it's holder and pull out one plug to plug in the dash and the other to phone, and if I am not using them then I will not have to see the cables hanging around. 

Hope that this helps someone out.
Next time I'm out on the trails I will be rocking out to commercial free music and not messing with CD's  
Let me know what you think.