Dimethicone is a methylated siloxane polymer capped with a TMS group at each end, and it is indeed used in hair products among many other things. These types of low-viscosity siloxane polymers, I believe, are what is in ArmorAll type products. Dimethicone itself is listed as being soluble in ether and in chloroform, neither of which one should be messing with, but does suggest you probably could find a suitable solvent if you were determined to.
I really think a warm solution of Dawn ought to do the trick. Rinse well, and consider you may also be washing some beneficial compounds like plasticizers out as well. I would put a non-shiny conditioner on it afterwards. (Please let me know if you find one that truly does not shine)
2002 E320 4-Matic
2008 Subaru Outback
2009 Subaru Forester