Have to first remove all the light bulbs, then put in a fresh fuse, if it still blows, then you will have to follow all the wires to find the short. If the fuse does not blow, then the problem is with either the bulb or the socket.
If this is the case, try disconnecting the ac compressor. If not that, it could be in the 'master switch' (turn signal lever unit). Does anything else go out along with the turn signals? Does it only happen when you turn on the heater or ac?
your heater core hoses could be plugged up and not letting much hot anti-freeze into the heater core then when you try to blow alot of air through it it cools the warm air off too much and you get not so warm air. try a cooling sytem flush