Adding Bluetooth to original Ferrari 456 GT audio system without wire running or splicing.

First off, many thanks to the OP for this thread, which contained all the info needed to get me started:
https://www.ferrarichat.com/forum/threads/1992-1998-ferrari-456gt-and-456gta-stereo-guide-ipod-and-more.222934/

My primary reason to install a Bluetooth adapter is to hear directions from Google Maps from my phone during leisure drives. Having access to your music library is actually a bonus.
This worked for a 1994 Ferrari 456 GT, with the original Ferrari-branded Sony XTC-F10 stereo head unit, Sony CDX-F10 CD changer, and Sony XM-F10 2 channel amplifier. For my car, the CD changed is powered (I can remove the magazine with no trouble, and the previous owner(s?) left 5 CDs in it), but it's not recognized by the head unit, which just displays "Reset" for a few seconds when the CD changer is selected as source. Otherwise, the head unit, amplifier, and speakers work fine.

All the modifications mentioned below happen in the left side of the trunk, where all the stereo components are installed, with exception of the head unit's faceplate in the center console.
To access the components you need to undo 3 screws. Two near the trunk opening that connect the cover for the left-rear headlight assembly with the CD changer cover, and the third near the furthest mounting point of the CD changer. You can then just pull the two covers out and put them in a safe location.

As I wanted to keep the whole original setup in place, I purchased a Sony XA-300 Auxiliary input adapter (you can find a few in "as new" condition on eBay and possibly some other sites):
https://maxmarineelectronics.com/product/sony-xa-300-aux-in-selector-free-us-shipping/
This provides an input for the CD (via Unilink cable), but in addition enables 2 new AUX inputs and one USB input (via an old-fashioned USB-B connector, though). I highly recommend that you buy one that brings with it the originally included Unilink cable in the box, as it is very hard to find a compatible Unilink cable today that is not a knock-off and that works with 90s Sony car audio equipment (learning this cost me $70...).

The Bluetooth adapter itself is an AudioControl ACX-BT1, which had good reviews for audio quality on Crutchfield and is also available on Amazon:
https://www.amazon.com/dp/B08XLVH2QT?ref=ppx_yo2ov_dt_b_fed_asin_title
My original idea was to possibly have a USB-powered Bluetooth adapter just connected to the XA-300, but as suspected, the USB-B connector carries no power (tested with an iPhone 16 Pro and a USB-C to USB-B cable I use to connect my laptop to the Yamaha NX-N500 powered speakers in my home office).
That meant I needed a power source for the AudioControl ACX-BT1 adapter. A close examination of the audio system harness in the trunk showed that there's an individual power connector with plenty of room to also connect the power wires of the Bluetooth adapter without splicing any cables. This connection is shown in the pictures below. The adapter has a third wire that is supposed to connect to a remote on/off switch. I had no use for it so just insulated its end with electrical tape. The two RCA connectors from the adapter are then connected to the AUX 1 input in the XA-300 and the XA-300 output to the input of the XTC-F10 head unit. This is after removing the CD changer cables, which should then connect to the XA-300 Unilink+RCA input.

At this point the adapter was powered and immediately discoverable on my phone's Bluetooth menu as "AudioControl BT1", so after connecting to it, and pressing the CD button twice in the head unit's face plate to select the "Aux 1" input of the XA-300, I could stream my Music Library and Google Maps audio, etc. from my phone. The quality was comparable to the best FM radio station on the FM/AM source, though the volume was about 1/2, requiring volume adjustment from the head unit's face plate for clear listening.

However, when I started the car the dreaded alternator hissing, which varies in frequency with engine rpms, was very audible and made audio listening rather unpleasant. This meant that there was a ground loop somewhere and/or interference between the Bluetooth adapter wires and the harness power cables.
The (surprisingly effective) solution was the PAC-SNI-1 Noise Insulator:
https://www.amazon.com/dp/B000K50HJE?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
I connected its input (male RCA connectors) to the XA-300 output connectors, and its output (box-mounted female RCAs) to the head unit's RCA input male RCAs, and all the hissing is gone at any engine rpms. I also made sure the Bluetooth adapter was well away from the harness power cables. The somewhat surprising result is that the Bluetooth volume (AUX 1) in the head unit is now just about at the same volume as the FM Radio stations and there was no obvious degradation of sound quality.

Overall, the Bluetooth-streamed audio is quite decent. It's not anywhere near the quality of the Burmester High-End system in my other car, but it's very listenable and as good as the Ferrari's FM quality, which is fine for a 30-year old car. Anyway, just thought I'd mention this solution, which leaves the existing system fully intact and is fully reversible, in case it helps other 456 GT custodians.

 

In case it is useful for those that are not interested in keeping the CD changer working or have not found a Sony XA-300, the diagram below shows how to connect the AudioControl ACX-BT1 to the PAC-SNI-1 Noise Insulator and finally to the Sony XTC-F10 unit mounted in the trunk.

The image below the diagram shows circled in orange the continuous power socket I connected the AudioControl ACX-BT1 power cables to.

 

Apparently images linked from my media album are not showing so trying again with direct attachments of smaller sizes and png format. (these at least show as attachments in the post Preview)

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