FM modulators have always sucked.

If you aren’t good with wiring, I wouldn’t run the risk of goofing something as it can translate to electrical gremlins down the road. If I can keep pushing to get a new HU, Crutchfield has a service where they will actually make the adapter harness for you and it is literally plug and play. Well worth $20 to me. I spent 3x as much time on cleaning out the old stuff (a hotrod FM modulator from back in the day) and removing the center stack and putting in the new cage to hold the new HU than the actual HU install (which only took about 10 minutes thanks to the plug and play harness).

 

Disconnect the antenna from the back of the radio. The FM transmitter will work fine. That's what I do in a couple of my old BMWs.

I'll follow up on the CD changer kludge this afternoon.

 

Examining that photo of the back of your head unit, I'm not sure that the blue plug is wired correctly. For example, it looks like there is a black wire going to pin 18 but the table below says that the pin is not used.

 

I'm with the OP on preferring stock-looking head units. I really don't care for dancing lights and remote controls. Cannot go wrong with simple functionality and buttons.

Can't wait to pull the Pioneer out of the Disco and get the OE head unit back in there. Hopefully it's not too fooked back there in the dash. PO even put a big JL subwoofer in the back. lol

 

Hopefully they used adapters for the radio connections. You want to see fooked, I had to unravel this mess under the seat.

 

OK, here goes. This is the plug you want to find under the CD changer beneath the passenger seat.



This next part is not 100% necessary, but it does make for a cleaner install because you do not have to cut any wiring on the truck, and it will allow you to remove everything as if none of this ever happened. I cut the piece in the white box off a junk CD changer and the part in the red box off of a donor truck. The plug in the lower left corner of the box is the same one you see peeking out from under the CD changer. I soldered the green to the pink, the purple to the purple and brown to the black wires back together, these are the power and datastream wires that will need to be connected for this mess to work. The wire above the green arrow is audio output of the Bluetooth module, I soldered those wires to the other colored wires described in the diagram below. This is the new audio feed which replaces the CD sound normally heard.



This is what it looked like what I was done. If you don't make the little "subharness" I made then you'll have to cut your wiring harness to patch in the audio wires from the Bluetooth module into the truck's harness, which feeds the blue plug at the back of the radio. I would not do this on the CD side of the plug, but the truck side. Side note, the first time I tried this I soldered all the wires from the bluetooth module to a blue plug with a few inches of wire on it and just plugged it directly into the back of the radio. This is when I discovered that without the datastream wire from the changer feeding the head unit, you cannot select CD from the front of the head unit. For some reason I dove under the seat to solve the problem however it could also be done in the dash by cutting the harness there to patch in the bluetooth audio feed right at the head unit, leaving the power and datastream wires still connected to the CD changer. Because the option of going 100% back to stock was important to me if this kludge was no longer desired, I made the subharness and did everything under the seat.



This is what the completed installation looked like, note the janky wire going into the center console. This is where my bluetooth module lives and gets its 12v power source. Not the best but hey, it's where I ended up.



When I did this in another truck I own, the one above where I had to yank out the Kenwood amp from under the driver's seat, I did a much cleaner install. Basically the same deal but I did a better job routing all the wiring. The CD changer has **** the bed in this truck so I'm back to square one. Someone with a far better understanding than me of how digital audio signalling works can probably figure out a way to feed the head unit some kind of false datastream to get around needing the CD changer altogether, but that's not me.



So here's a mock up of how to wire this. There are a couple of different ways to do it but this is the crux of the idea. Hopefully the image autoscales because it's pretty big in case you want to print it and read the text.

 

I got an FM transmitter cigarette lighter adaptor thing. It works flawlessly plus I can fast charge my phone on it. I've had for 2 years and the last 20k kms and I've never had any issues. I bought it off Amazon.

 

As an aside can you post some pics of the actual navigation running. I can imagine it being more helpful than a paper map unfolded blocking driver view but not nearly as useful as today's mapping apps

 

This is kinda why I stayed with a “normal” single DIN HU: the phone stuff is so good these days, or I actually mount an ipad for longer trips, the original unit is more a novelty than anything. I think this is truly aesthetic for the OP and many others (and I can’t argue as the original HUs do look very good, I just went for the easy button).

 

Thank you for saving me hours. I have the same GROM unit, and the Navigation system, and the unit does not recognize the Grom. I'd already spent a couple of hours troubling shooting to no avail, and I came here to see if anyone had more info, and found this very timely thread.
I may try and patch power to the unit inside dashboard, and see if that can wake it up.