I am installing a "powered" converter to operate the flat 4 wire trailer light wiring. Using the open socket in the spare tire compartment, I have been able to identify the wires for the 12 volt feed, ground, stop light and the right and left turn signals. But the blue wire that is supposed to activate the trailer tail lights shows no voltage when the wagon tail lights are on. Is there a switch or a fuse somewhere to activate it?
thank you in advance!
Carlos

A powered converter is definitely worth it! The converter draws current from the red wire in the "trailer socket", which has a 15 amp fuse on position 9 in the fuse box in the cargo bay. I used the ground wire, the stop light wire, the two turn signal wires, and for tail lights i tapped into one the "parking light" wires (the white one). Connected my little utility trailer with its new LED lights, and it works like a charm, no error signals, no problem and plenty of light.

NOTE: The converter instructions state to draw power straight from the battery (with a fuse of course), and this makes sense because in an emergency you could turn the key off and yet could still run the flashers for example.

Apologies for this egregious mistake, i just rechecked my diagram of the Volvo trailer connector, and it does not use tail light wire at all, it uses both "parking lights", both turn signals, stop and ground. It must be a different system altogether.

Klaus, thank you much for your observations. I have a good ground but that blue wire seems to be dead.

Let me do a reality check with you guys, here is a description of the wires in the “trailer socket” located in the spare tire compartment (I just copied it from another post in this site);

Blue +12V in key position II via fuse 11D/15
Red +12V at all times via fuse 11D/9
Yel from brake light relay. +12V with brakes applied.
Pink rear fog light (may require software to activate)
Black Ground
Gray R parking light
Grn-wht R directional (connects directly to lamp socket)
Green L directional (connects directly to lamp socket)
White L parking light
Blue Tail lights (connects directly to lamp socket)

Volvo sells a harness (Volvo Part #114827) that connects to this socket, goes through a powered converter and comes out with a four wire trailer plug. I have been studying the harness that connects to the socket in the car to see what wires Volvo picks out of all those I listed above (here is the link to the pictures I zoomed into to see where the pins are https://www.matthewsvolvosite.com/forums/viewtopic.php?t=66204)

There is a ground connection (large white wire), the stop light (red wire) and the left and right turn signals (yellow and green wires), no problem so far. So, there are only two wires left to connect, one for the 12V feed and the other for the tail lights, correct? To my surprise, Volvo does not pick either of the two 12V feeds to power their converter! The two additional wires in the converter harness connect to the tail lights (grey wire) and the brown wire connects to the Right parking light. My guess is that the tail light wire in theory provides the juice and the brown wire the signal for tail light on?

But if Volvo is tapping into the right parking light, there must not be enough of a current draw to trip any "light out" indicators anywhere. Since my "tail light" wire has no signal, i guess i am going to use the R parking light for the tail light function and see what happens. i will power the converter from the red 12V wire. I promise i will report results!

There are no external switches, and just one fuse on the battery lead. Re-check the ground when probing the blue wire.
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Keeping it running is better than buying new