Board Thread:Questions and Answers/@comment-7411320-20130324045348/@comment-66.68.34.127-20140909130706

Trinity most likely found out where Dexter lived after visiting his old apartment, which was now Debra's apartment in by this point in season 4. Debra would have had information on Dexter's address lying around somewhere. Otherwise, he likely did an internet search during one of the time lapses and found his public address.