No air bricks in old extension, cause of damp?

Hi, I recently bought a house without survey, stupid I know, but I really wanted the house and didn't want to be put off. I live to regret that decision, ive ended up ripping out all internal walls upstairs as they was a very thin breeze block and weren't stable. Anyway after 4 weeks hard graft after work and weekends I'm now downstairs ripping out. Ive come across damp on what was once the back wall of the property and is now an internal wall in an extension. Ive ended up taking off the plaster and finding an air brick (pictured). Am I right in leaving the vent exposed and could this be the reason for damp as it was plastered over. The other side of the wall on the other side of where the back doors were hasn't got an air brick and has damp, should I put an air brick in there? Also the extension it's self has No air bricks around the perimeter. Should I install air bricks as there is rising damp around the whole kitchen area. Don't want to just plaster it and hope it goes away! Thanks in advance. Can't upload picture as it's too large, hopefully people can help through description.