Reinsulate your house! We've had ants in the house for as long as we've been here. Could never find where they were coming from. This summer, I've been tearing the siding off the house, tearing the sheathing off, adding insulation to the walls, and resheathing and refinishing the cedar siding to put back up. We got to the bedroom walls and when we took the lowest course of siding off, finally found the ants.
We hit the area with raid and haven't seen an ant anywhere in the house since.