Get some suds going with a scrub brush and go to town. I've found this works best for me to get out the fine dust and grime. Works great with a shop vac if you let the carpet soak a bit.
Think of a carpet at home. If something's been walked on for months, you won't get it clean with a simple vacuum. Gotta scrub it a couple times and the suds will help the vac suck the crap out.
I just did this to my 02 wrx and managed to get the the 13yr old carpet clean. Took a couple rounds of lather, rinse, repeat.