Garage Extension on side of House Damp ?

[MattLiving]

I have moved into a recently renovated property (Semi-Detached 1930's) which as a new garage extension to the side. The garage extension is modern breeze block to outside wall and as one wall adjoining side of property such that x2 of the original subfloor vents present in the garage ( which are mostly covered by plasterboard). Previous owners have put plasterboard up on all of the internal wall and across much of the lower section you can see damp patches (see attached image). Feeling under the bottom of plaster appears slightly damp and my initial thoughts were condensation as the garage is pretty much un-ventilated - other then that some kind of rising damp. Dampness possibly worse at location of subfloor vent which is just visible under plasterboard but across most of length.

Image also attached of outside garage

I would like to improve the situation and wondering what the best way forward is, I assume plaster will need to come off. Would like a this wall to damp free so I can use garage as workshop.

Previous owner clearly covered the damp patches with building materials to hide which obviously will not help any moisture situation.