Reading the first post, you say that the caravan indicators are dimly lit all the time. Have checks of the towing socket and towing system been made? The assumption is that the Volvo towing module is causing this by sending out a checking signal as it "looks" for a circuit through a conventional filament style bulb. That signal is not enough to light up a conventional bulb but is enough to cause a sensitive LED to illuminate.
The problem is caused by your car being designed for conventional filament bulbs and your caravan being designed without a thought for what will be towing it. If it has LED indicators then another problem that will arise is that the bulb failure monitor for the indicator circuit probably won't work - and indicator failure warning is a legal requirement - does the trailer indicator lamp flash on the dash in time with the indicators?
One way around your problem may be to modify the Volvo. The trailer indicator circuits could be rewired to switch relays fitted between the tralier module and the towing socket, perhaps with an audible monitor to warn of failure.
Or you may find that a more modern design of towing module which has LED compatibility is available for your car. UK company Right Connections (manufacturers and distributors of towing electrics) are experienced with this type of question.
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