Indicator not working

 Have you tried cleaning the contacts in the bulb holders for the indicator.

I would still try a new bulb

You need to find out whether the fault is in the car or caravan. Can you connect your car to another trailer/caravan/trailer board? Have you got an indicator supply in the black socket on the car? If yes, how far into the caravan does it get? 

Thanks for getting back to me. We now think it's the relay or other component that isn't putting enough voltage to the indicator. 

Beware checking for volts.

A  corroded contact may well pass enough volts to show 12V on your meter, but it may not pass enough current to energise the bulb. You need to test with a bulb, soldered to a probe, with the other side earthed. Once you’ve made up the tester, keep it in your tool box.

Good point. Many moons ago, I was asked to look at a none working interior fan on a car. Had volts to the fan when switched but wouldn't work .... but fan would work when 12 volts from the battery was applied directly. Fault eventually traced to a plug/socket through the bulkhead .... a wire had all been cut through & had left about 2 strands.