Open the bonnet, pop the back covers off each light (where you access to change bulbs) and position a hair dryer to blow warm air into the light. Go for a cup of tea. Repeat for each light which needs it. Alternatively, remove whole light unit from car, remove any fixing screws and put unit in oven (REALLY !) for 10 minutes at about 100º. Pry the light apart. There will be loads of stringy black (usually) stuff holding them together which is Caulk. Add a load more, or silicone sealer, pop back in oven to reheat and then push back together, refitting any fixing screws. Then hope it worked as you won't know until it either steams up or it doesn't. Alternative #2, scour eBay, net spare part search sites, or the local scrapyards for replacements. Alt #3, pay through the nose for new.