If it's a aircon car then it will have a cabin filter at the inlet to the heater box, before the fan. Itwould be a good starting point to remove the filter, get some compressed air into the aperture it came from and blow out any debis. Does the heater fan run when you turn it on? If you can solve the problem without removing the heater box it would be good as it is a mammoth task to strip it out on the K12.....