An easy but messy way is to blow compressed air or nitrogen in through the drains and the vents.
Remove the fan blower and pump the compressed air/nitrogen through the coil.
This can cause the cr*p to be blown into the cabin - and will take a bit to clean up. But if you pop clean tissues/rags over the vents they will collect most of the ****.
After it's cleaned out use a sprayable eculyptus oil or tea tree oil (ask Bursons for ERG ABS (anti bactrial spray)) - spray it through all the vents and the drains. These kill the bacteria and stop the gunk building up. You could use wd40 or the like but eculyptus or tea tree is far better smelling.

