Before going aloft it would be wise to make sure the power is at least getting as far as the base of the mast. There will be a connector of some sort there and you will need to access it and, either using a meter or a 12 volt bulb on wires, to test that when the switches are on the meter shows 12V or the test bulb lights.
No need to disconnect anything and if power is there then the intermittent fault is upwards!
A meter only costs a few dollars or you can make a test bulb by buying a small bulbholder, fitting a pair of wires and a 12v bulb. Or, if you are good at soldering, then just solder a wire onto each bulb contact.
Either way you will surely have need for it on other occasions.
If both lights are out then look for a fault in the common earth return wire (if there is one).