I had a leaking container of used oil in the trunk that seeped into the carpet covering the trunk floor (over the spare tire). I'd use some sort of soap, but the carpet is attached to a peice of cardboard-like wood material and I'm afraid water it will destroy it. Any ideas?

 

I had one too-in different car though, without cardboard. It was synthetic oil and spill was BIG. I took carpet out and washed it with washing detergent and garden hose-after about 5 runs carpet was still slightly greasy and I left it like that.
Try to treat stain with Simple Green, if you cannot use hose down technique and stain is small. May be try steam cleaning with detergent?
Any other ideas?