Remove all the dry dirt ( no short cuts here)
equipment req.,steel crevice tool for your vac,hardbrush,dogy brush and a brush comb (chemspec cat. page 22) access to an airline, with a blower nozzle
will help a lot on the bad cars
Use all these tools in combination as needed,ie vac ,brush,vac,comb,vac ,blow,vac , etc.
Carpets still looks dirty?
Black,gray and midgray carpets use blackdye,as supplied by autosmart or autoglym (both on the web)
Any other colour use carbrite aerosols (01733 384150)
But why not hwe after you removed the dry dirt properly?
What are you going to do with the seats?