Yes. I have factory navigation in my car. I have a double din. They both work exactly as intended, except for the voice on the factory navigation unit.

Completely sure. Your installer is flat-out, 100% wrong. There are a couple reasons they'll say it cannot be done - they've never tried, or they tried with the wrong parts. The right parts are listed in the first post of this thread - JDM trim for 2003-2004, or JDM navigation trim for 05-07. The Metra kit (the *ONLY* other trim kit for our cars) doesn't fit correctly without modification. A proper audio shop can modify the metra kit to work, but it will not look factory at all - because it's the Metra kit. Regardless of the trim, you will lose the factory navigation voice without taking extraordinary measures, but the nav works just fine with factory nav installed.
If the shop still argues that it can't be done, give them a printout of the matching year from these two photos along with the message that you're taking your business elsewhere.
Here's the proof - both 03-04 and 05-07:



So why do they say it can't be done?
Because, both the Metra kit and the non-nav JDM kit do not fit with the factory navigation without modification.

The problem is that the non-navigation one is about 1/8" too long, and overlaps the navigation panel.

So what modifications are necessary?
You need to remove the upper section of plastic above the plastic shoulder/rib cleanly. Alternatively, you can send it to me and I'll do it for you, free of charge (PM me for details).

The process is detailed in this post:
https://g35driver.com/forums/audio-v...ml#post5858335

For 03-04 vehicles, refer to this video for modifying Metra trim to work with factory navigation:

For 03/04:
The Metra 99-7604 material match is very good, but the buttons are gray soft rubber. Also, the paint on the Metra kit is not as durable as the OEM/JDM piece and scratches off easily.
The JDM trim matches perfectly.
The 99-7604 costs about $150, the JDM about $320.

Metra on the left, JDM on the right:


For 05-07:
The Metra 95-7605A material matches extremely closely, but the buttons are still off, and there isn't the smooth aluminum edging around the trim.
The Metra 95-7605 surrounding material does not match, and the rubber buttons don't match.
The JDM Trim matches perfectly (as it should).
Prices are about $100 for the Metra 95-7605, $150-200 for Metra 95-7605A and $300-350 for the JDM trim.

Metra Aluminum-Look (95-7605A) on the left, Metra Plastic (95-7605) in the middle, JDM on the right:

Yes. The long version (with pictures) is here, courtesy of docron1. This is necessary for all 03-04s that have dual, because the Japanese version of our cars did not have dual as an option in 03-04, so the replacement trim and a/c controls were never made with dual controls on them. For 05-07, it is not usually necessary, as all my recommended sources for the trim and a/c controls price the dual controls the same as single, so there's no excuse, unless you can get the single for a much better price.
Here's the cliff notes version for the more confident: (heavily modified instructions from PeterUbers' original write-up)
You have to grasp the volume **** firmly and pull on it hard with your Man Hands®. Do not pull it any way but straight out. There are clips inside holding it in.

Once this is out, the rest disassembles down to the circuit board easily. The microswitch for the DUAL button is attached onto the board via two tiny little solder joints. Using a fine-tipped soldering iron, heat up one side while lifting and wiggling the button gently with a pair of tweezers. Alternate between each side to carefully remove it. Solder it in the same location on the JDM A/C controller. The LED is even smaller and is removed in the same way. Remember to keep it pointing in the same direction when installing, since it is a diode, and only lights up with the correct polarity.

You Tube

Basically, get somebody who's good with soldering and can build this for you:

You can get them out fairly easily, but your old CD player will be unusable unless you're very careful:

The G35 has an antenna built into the rear windshield. This requires that the signal from that small antenna be amplified. This amplifier is not just activated by the factory radio, the way most power antennae are. It is powered by the radio. Because of this, the blue "Power Antenna" wire on the 70-7550 harness or ROEM-NIS2 has to be connected to a true +12v power source, not a +12v signal. The easiest way to do this is to connect the Blue (Power Antenna) and the Red (Accessory Power) wires from the 70-7550 or ROEM-NIS2 together. This powers the antenna amplifier the way it was with the factory radio.
Here's the diagram from the factory manual which shows how the antenna amp receives power solely from the radio, rather than any other source. This can be used to convince an installer to do this install right the first time.