Are you using the Volvo cable harness and trailer module with software upgrade? If not, that might be part of your problem. If you are, it may be that pin 10 (you talk about wiring both sockets but pin 10 is on the single 13 pin socket) only becomes live once it senses a load on it, such as the fridge. So testing it with a multimeter won’t show any voltage.
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Simon J
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