A voltage-sensitive relay is the obvious solution to your problem
Remember that the car never knows what type of trailer is connected and thus never knows what pin 10 will exactly be used for in the trailer.
Be it for releasing the electric trailer brake, for powering a fridge, charging a auxiliary battery - all of these are possible.
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XC90 R-Design MY2009, Black Sapphire Metallic. HP-Sound, RSE, Nav, Tel, ParkingCam, BLIS, ParkingHeater, RestHeat, Removable Towbar, Summer: CRATUS 20x8 on Pirelli Scorpion Zeros 255/45, Winter: NEPTUNE 17x7 on Continental WinterContact 4x4 235/65.
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