I do my seats/carpet using a steam cleaner (about £60), which is superb. It leaves the seats a little damp. The steam gets rid of everything, leaves the upholstery soft.
Its a good purchase as you can do household carpets and sofas etc also with it.
Mind you, what I found steam cleaning, which is probably true of all these types mentioned is how clean it gets depends on the time you take, there are no wonder substances etc.
I used lots of towels, usually white so you can see the dirt, and run them across the upholstery slowly but firmly, they soak up the moisture and with it the dirt.
Funny smells, like dog smells, can be gotten rid of by using white vinegar mixed with water.
But maybe get a lot of towels, all a steam cleaner or above mentioned products do is break down the dirt, you have to remove it, which is the time consuming part. Id recommend lots of towels for that bit.