I've found the biggest culprit for nasty niffs in any car I've had is, more often than not, the mats and/or the seats - particularly if they are fabric seats.
Car mats will clean superbly and be 'fresh' if you wash them with a wet/dry vacuum cleaner such as a VAX.
You will be stunned at the amount of grit/sh*t/dirt that comes out of them. I have a narrow cleaning-head for the job rather than a full-width one for domestic carpets in the house.
This small cleaning-head is about 4" wide, and is brilliant for leaving stripes in your mats (like a mown lawn!).
People sweat. So washing cloth seats is a must, and can be done using the same small cleaning-head and the VAX this will eliminate a lot of smells.
The other bit that gets neglected is the headlining. More care needs to be taken here as it can be more fragile, so I do this by hand with a sponge and hot(ish) soapy water
A big plus for cleaning the Air Con with those cans but I found that it needed to be done several times.