Hey Guys.
I gave it a day but failed to do Aux line in without a PIE adapter. It is possible as a guy on ebay is doing it.
07 Ford F150 MP3 6 CD Radio Ipod AUX Sat Line in 7L3T18C815AG 30 days warranty | eBay
Seller: factoryradios | eBay That link will work when the item fades away.

I'll post everything I found here for future posterity.
The 16 pin connector on the back out our factory stereos is considered a CAN BUS connector or MSCAN (2005 and newer).

The first number in the model # of your stereo indicates the production year:
53CT:
5= 2005
3C= F350?
T= Radio


I failed to find a pinout for the bus connector. If someone has this pin-out, please post it here.

Those that have successfully wired a line in to one of these newer bus connectors on other manufacturers' vehicles had to jumper 2 of the wires together. Here are a couple examples of how people have done it in other vehicles:




I tried all three of those and a few other combinations with no luck.

There are a few good instructables on this topic where people just solder into the CD or Radio lines on the PCB.
Disassemble
This seems to work pretty well. The higher signal level of the input overrides the CD or Radio signal and all you have to do is unplug your MP3 player and it goes right back to the original input.

This is the method is used by the above mentioned ebay seller in pre-2004 stereos that don't have an Aux button. If you look at his auctions for Rangers and Explorers he actually says in the description that he is inserting the signal on the pre-amp side of the board. However he has managed to wire it in to actually use the Aux button on the 2005 radios so he must have figured out out to tap in on the PCB.

Here is a pic of the bottom of the board at the 16-pin connector which is probably where it would need to be soldered in:



I would guess that the line in would need to be soldered to the "DJl1, DJL2, DJR1" solder pads, or maybe even the "Mono1" and "Mono2" pads. Then there would probably need to be a connection between the ground of the aux in line to either the "WAKE" or "CANH" or "CANL" pads to activated the signal and allow the radio to recognize the input.

The ebay seller puts the jack right in the front panel of the radio. I took mine apart and there is infact plenty of room for a jack in there:

a hold would be drilled through the front and a PCB mount jack would be glued in. There is 3/8" of room which is plenty for most of the PCB mounted jacks I've seen. A wire would then be ran from the jack to whereever the aux in is wired into the board.

I know you guys probably think I'm crazy for going through all of the trouble to get the board out. But I had hoped I would find nice little "AUXL" "AUXR" "AUXG" pads on the board.

Here are pics of the rest of the board and there are some other juicy looking places that might be the Aux in:
The "PTAIN" and "RPAIN" in this pic:

This is the front panel connector point:

Here is the CD Input where one could just tap into the CD input as described in the instructables:

I think this one is the Radio part which could also be tapped:


I gave up at this point, as I couldn't find ANY INFO AT ALL on the internet. Which is why I'm posting this here. Perhaps some other enthusiastic chap can take it to the next step and enlighten all of us. Or.... IF someone has bought or was thinking about buying one of the modded units off of ebay with the jack, you could be the receiver of MANY MANY REPS if you opened it up and told us where he is soldering the connections... ;-)

After all that I'm going to break down and buy a PIE connector, or just go aftermarket. The cheap Windows CE 6.0 Double DIN Chinese GPS units on ebay are looking quite appealing with the steering wheel control customization and the ability to load Garmin XT TomTom and any other WIN CE programs on them (with a little hacking).

One last reference question for posterity:
PIE sells 2 different versions an "X3" version or "X3-FRD/P2" which is only $25 and the version with the actual beakout box which is $86. Anyone know if the cheaper version works with 2005 and newer radios? Which is the proper model # for 2005 and newer head units?

 

You won't find anything on the CANBUS protocol. You can look at the ISO standards, but they're pretty expensive, and allow for a lot of proprietary signals.

Simply grounding the CAN lines might cause a world of trouble, as the CANH and CANL lines transmit CAN packets by changing voltage. Ground one and you've introduced noise into your whole CANBUS (the same CANBUS that runs your ECM and PCM).