Adhesive remover (goo gone, acrysol etc) does work quite well on really dark stains to help remove them. Just make sure you get it all out or it can slowly eat away at the carpet.
For off the shelf cleaner I find that the turtle wax foam interior cleaner works well. Or if you get a spray bottle of water and mix it with steam cleaner soap, spray it on, then go over it again with another spray bottle of just water, scrub then vacuum with a wet/dry vac it works like a half assed steam cleaner. If it's really bad and depending on where you are (you wouldnt want to do this in the middle of winter) use a hose and put a fair amount of water on the carpet (don't do this to your seats), use the turtle foam or steam cleaner soap or adhesive remover OR all of the above depending on how bad the stain is, scrub, vacuum. It's the poor mans steam cleaner lol works good though! Just make sure you vacuum out as much of the water as possible. You don't want it sitting on the sound dampening and rotting out your floor lol.