Check the a/c filter and replace, it’s usually behind the glove box: your car manual will tell you, if it is old it will be very dirty and contaminated. A new one is about $20.00
hey duggy, tried this out at the weekend, i also cleaned the whole interior of car including washing the seat fabrics too and i have got to say, i've cleaned the car fabrics before and it didnt work, but combined with a clean new a/c filter it has made so much difference. thanks so much now the car wont smell so bad when people are looking to buy it. :)