WE HAVE A DAMP PATCH INTERNAL WALL

We've bought this house which been previously extended, the damp patch is appearing on one of the old external wall which now is internal wall. When we moved in it appeared that all the old external walls have got damp patch which rises to about 70 cm high, so we had builder to relay the extension floor and put the damp proof membranes in, however it seems one wall has still got the problem. There is no door and windows leaking, and no electrics to either side of the wall, Please anyone can help to treat this problem?