tip I had, and works, use a household disinfectant spray (domestos or similar) and spray into the inlet, run the fan on low for a few seconds to pull it round the system, leave for a while to do its biz. if its really bad you may need to do it a second time.
Sorted.
Just make sure it has NO BLEACH in the spray, that wont do yer upholsery any good. I did nottice recently that some fabric fresheners have anti bac in, those would be ideal