6. I (write) you a postcard if I (have) your address.

6. I would have written you a postcard if I had had your address.

В условных предложениях третьего типа (Type 3) в придаточном предложении (if-clause) используется Past Perfect, а в главном предложении (main clause) используется would have + Past Participle.

Ответ: I would have written you a postcard if I had had your address.