Mine's hidden in the cupboard next to the deck. You really have to look to find it.
Failing that, open the cd door and pull off the rubber side pieces. There's 2 screws on each side under them.
Be careful sticking your hand in there if you're plugged in to shore power...

 

On my 320, the AUX input for the XMD3 was bundled off to the side of he receiver. I expected the RCA jacks to be mounted directly to the back of the receiver, but instead there were two cables that were coiled up with other cables and wadded into a space just to the left (as you face the receiver) of the receiver. They will have rubber protective caps on them. Take the power panel cover off and reach up behind the XMD3 and then to the left of the receiver. That's where you should find them.

I used a flashlight and an inspection mirror in my quest and they were very helpful.

 

On my 32 the input jack for the I pod is on the side of the cabinet that the radio and breaker panel is in. when I bought my boat and found out that all 32' were wired wrong and that you have to remove the radio and switch two wires around. Unfortunately this is a big project. all of the wires are clamped to the walls of the cabinet and the radio will not come out unless you remove the breaker panel and remove all the clamps. There is a digram on line that showes the wires to reverse. Its no a fun job.

 

My '05 320 didnt have an jack installed where you could get to it. Remove the panel for the AC/DC switches (right below the stereo) to get better access. The front panel on the Clarion has little plastic coverings that come off revealing the screws that hold the stereo in place. Remove those screws and the stereo will slide right out towards you. On the back of the Clarion are the hookups for an Aux input. I went to Radio shack and bought a 3.5mm jack and hooked it up ( i believe it was a red and black hookup on one side and the 3.5mm female on the other). I bought the 6foot cable so i could run the connection a little farther than having it fix mounted. It took about 30 minutes total but what a great addition to the sound system. Hope this helps

Mike