Spray bottle of 3/4 full strength simple green and about 5,000,000 rags.

I sprayed it good and let it sit for a few minutes, but it cleans really well. Easier to do in sections. Here is a pic of my '90 SHO when I was doing it. I got 1/2 way through and sold the car to someone that drove by, so no finished pics, but here are some process pics of my "quick clean"...
Before closeup inside fender:
After closeup inside fender:
Overview Pass side inside fender, front, & headlights finished- I fixed one rust spot in front of the airbox, but the passenger half was unpainted- it matched perfect!
Yes, I know my plugwell seals were shot. The new ones were in the trunk!
A lot of the plastic covering I pulled off of the '97 SHO so I could get in the nooks and crannies. That SHO was the hardest, because I bought it with 130,000 negected miles, adn it was trashed. I pulled the intake to clean the tank and runners, to that helped, and it took about a weekend to get to that point. The 2004 I owned since new, so my **** retentive-ness was merely maintenance, and the Mustang was less complicated to clean up.