To confirm I've got a 2019 custom limited panel van with a Sync 3 head unit... like most people I have the wiring in the existing loom for the rear drivers side speaker.
I have activated the rear speakers and fade functionality using forscan (i can post details of this if people are struggling with this).
Unfortunately, it appears that the passenger's side loom doesn't have the rear speaker wiring like the drivers side. However, the wiring for the rear speakers does go all the way through the loom until the final connection to the loom running down the passenger side. This connection is found at the base of the passenger's side A pilar.
For access (which isn't great) you will have to remove the glove box and housing. Step by step here....
https://www.youtube.com/watch?v=tunNSZM2oaAOnce you have all this removed, you'll have a large wiring loom connection directly in front of you.... this isn't the one we need but you can see the speaker wiring in this connection (white and green / brown and yellow - arrows point these out plus the arrow at the back is the general location for the connection we are looking for).
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Tucked away behind this connection (back left) you should be able to locate the passenger side loom running from the bottom of the A-pillar and connecting into the other wiring.
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When you separate this connection, you can clearly see the wiring for the rear speaker stops here. Test it to get some confidence!!
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Then you have to cut these wires from the back of the connector and connect in a new speaker cable to run alongside the passenger side loom to the speaker location.
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Its really tight so a bit of a faff to connect up!..
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