This is a DIRECT copy/paste from my g35 forum where I wrote this how to. I know most isn't relevant here, but figured I would share a bit of my work even though it's not that great.
So, like many I decided to go with the PAC unit for my Ipod, but since I did not get factory navigation I also use a TomTom. I know, it's ghetto, but it works great. You could do this install with any GPS of your choice. Please don't make this a TomTom vs Garmin debate.
First, following the steps outlined on this forum for factory radio removal, I removed the factory unit. I'm not going to go into detail how I did, search for it, there is a great walk through already.
Here are the parts I started with:
PAC Unit, TomTom hardwire cable(ebay), Ipod hardwire w/ charging option(ebay), various suction clips/mounts(ebay), toggle switch(walmart), zip ties(walmart).
Other stuff you will need: Hot glue gun, wire snips, inline wire strippers, soldering iron(i love my Snap On butane), solder, towels, electrical tape, and other misc stuff I'm sure you will find you need.
Here is a picture of the Ipod cable:
Here is a picture of the TomTom cable:
Once the radio was on my workbench, the first thing I did was drill a hole and insert the toggle switch. The toggle switch is for the power to the devices, as it is NOT GOOD for the devices to be constantly charging when they have a full charge already.
I then drilled a large enough hole to fish my Ipod and TomTom wire through. Note how much slack I left hanging out in case you choose various mounting options.
I then soldered the two grounds together and the two positives together from the Ipod and TomTom cable.
Next step was to wire the positive to one side of the switch and have a loose wire for now hanging off the other side of the switch(give yourself a couple feet). Then cover the whole thing in hot glue so the switch doesn't move and it keeps the contacts protected.
Reassemble the dash going in reverse the way you took it apart. When you get to the shift boot piece, use your inline wire strippers and strip the green wire. Attached your loose wire from the previous step, solder it, and electrical tape it.
Finish reassembly and try out your new hardwire setups!
Lastly, throw out that junk FM transmitter and cigarette lighter adapter!
Overall, it sounds great, enjoy!