I see...I like to usually start with warm (closer to hot), soapy water. I will use a rag and put a few drops of dishwashing liquid (breaks up grease) in the water. 9 times out of 10 this will get you off to a good start.
My methods take lots of time, although it just about always works. This is what I would do, because i get tired of buying products that are good for only one specific thing.
On the other hand, i'm sure there is something out there that will specifically take care of this mess for you, but I don't have a clue what it would be...
Ideas:
1.) See if you can make a friend that works at some auto dealership...better if its someone who cleans/details the vehicles. They have all kinds of tricks for making the used cars look like-new again.
2.) Go online to the Mothers, or Meguiars forums (should be easily found from each company's homepage)...see what you can find out there about the various detailing products. The lists of products and uses is exhaustive!
3.) Sometimes, when I'm really determined to get things clean, I will completely dismantle parts of the car just to get one part clean and get the crevices around it.
4.) Sell me
the speakers....and I'll come to you and do the detailing