There is no Aux unit on the back. Here is a solution that I installed a couple of days ago. Works. Great.
You definitely can wire an auxiliary input into a pioneer DEH-1400. The design of this unit is highly modular and thanks to the engineers who designed it there is an easy, straightforward way to add an aux input to the unit. The way to do it is to decouple the the AM/FM receiver stage from the volume control/Amplifier stage and inject your MP3 audio signal there. If you have experience taking things apart, know how to use a soldering iron and are good at basic electronic wiring you can do it. It does take some time but for me, it was worth it.
Look for and down load the service manual for the deh-1400. I saw it on a number of sites and at least one of them lets you D/L for free. This has a schematic and will help you understand everything described here.
There are spare (unused) line level audio inputs to the Volume control (VC) chip but I was concerned that I would have to somehow re-program or spoof the system controller to tell the VC chip that the spare is being used, so instead I just decoupled the AM/FM receiver line stage from the VC chip. This is easily done by desoldering and removing jumpers JP96 and JP100 from the circuit board. You will see the jumpers when you take the unit apart, they aren't shown on the schematic or labled on the PWB layout drawing. These jumpers connect the FM line out to the input side to C311 and C312. Once the jumpers are removed, you now have a usable stereo line level input to the VC chip that utilizes all of the audio equalization and amplification features of the unit.
So how do you select between AM/FM source and Aux. input source you ask? Well, I just took the brute force approach and wired in miniature Double Pole, Double Throw (DPDT) Switch. One position joins the FM stage to the VC chip (like the jumpers previously did) and the other position connects the Aux. input to the VC chip. A convenient space near the head unit (on my 15 year old truck) was available on my dash to mount the switch and 3.5 mm stereo input jack. (sweet).
Another way to do it is to use a 3.5mm input jack that has normally closed connections which open when the plug is inserted the Aux. jack (like GC electronics part number 30-574). (this saves a second hole in the dash and a switch that you have to flip).
I used twisted, shielded pair wiring for all wiring, (shield used for audio ground) I enlarged an existing hole on the back of the sound deck (between the heat sink and the line out jacks) to run the wiring out. I picked up audio ground at the ground jumper for the VC chip (were C307 and C309 are connected in common). The solution sounds great and works good. Just run the radio in FM mode, plug in your MP3 to the Aux. Jack and ignore the tuning display. (or tell yourself "wow, every station is getting music I like ... and commercial free too!)
Notes on Input volume level setting and input impedance: If you are plugging your mp3 player directly into the Aux. jack you will probably notice that you should adjust the volume level on the mp3 player in order to match the volume level that is fed from the AM/FM receiver to avoid a sudden change in volume level when switching between sources. If you are getting fancy (like I did) and putting a docking station into your vehicle or you just don't want someone plugging in their mp3 player at max volume and making you have to then turn the radio volume down, you should put an attenuator circuit in line with the Aux. input. The attenuator circuit that I used is a simple home made (dual for Left/Right) resistor divider network. I used the same resistor values that pioneer used in their design coming out of the AM/FM receiver stage. (2.7K Ohm for the series elements, 1.6K Ohm for the shunt elements) I used 1/4 W, 2% metal film resistors. This will provide approx. 4.3dB of attenuation and should match the line level out of your MP3 (or the earbud out at max volume setting) to the volume level sent out by the FM stage thus allowing smoother volume transition when you plug the MP3 player in. The resistors also provide some isolation/matching between the (relatively) low impedance of your MP3 output and the VC chip. (The attenuator is really optional but I like having the volume levels matched when I move between sources).
Good Luck, worked for me.