firstly i'd take the lamp off, empty out any liquid if possible, and stick it in the oven for about 15 mins on 120 degrees, then take it out and squeeze it together all around the edges, re- fit it, then drive with your headlights on with the sidelight unplugged from the lamp so the air is circulating, once all the condensation has gone, stick the sidelight back in and see how you get on, total cost £0
if that fails then get a new lamp, but it worked for me