Pin 10 is the fridge positive output within the 13 pin socket. This pin also activates a relay within the caravan to link the caravan battery to Pin 9 and charge it from your vehicle. Pin 11 is the negative earth return for the fridge. Most cars don't require a load or connection to activate Pin 10 - it is usually ignition switched. However, some VW cars and perhaps Volvos do have an option to program the output to be dependant on a trailer or caravan being connected. Many OE fitted towbars (BMWs in particular) don't have Pin 10 connected at all - the excuse I've heard being that it apparently isn't used on German caravans. Pin 9 should be constantly powered with the caravan connected and runs the battery charging supply and also the Alko ATC electronic stabiliser built into some modern caravans.
I'd use a simple 12v test light to probe between Pin 10(+) and Pin 11(-) on your Volvo socket with the engine running and hope to see an output which should switch off when you stop the engine.
If you see nothing, perhaps the system requires a load - so you could connect up your caravan, start your engine . . . . but where to check? Don't rely on the light in the fridge as it is powered from the caravan, and they make no sound so listening is no good. Most have no lights on them either.
However, if you have a volt meter built into the caravan or have a meter to connect across the caravan battery, you should observe a charging voltage of around 13.5 volts, and this is reliant on Pin 10 working correctly. If the caravan battery isn't charging from the car then you need to check the Pin 9 output. If that's ok then its highly likely that Pin 10 isn't working - so no fridge supply and no battery charging.
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