Second to Nial's cheap pressure washer and JPS' Gunk. You can usually find some new or rebuilt bottom-end electric pressure washers at Harbor Freight, Northern Tool, and other discount outlets. Often name brand type stuff, and some others. Not an expensive investment ($60-100) and the lower pressure models (1200 -1400) are great for general underside and engine bay cleaning. I keep one hooked up to the shop hot water line and wash the car mechanicals and engine bay with it after a race. Do the wheels and tires as well. Use Gunk, Wal-mart brand or generic auto-parts store brand degreaser. A CAUTION!!!! Ran into some cans of Wal-Mart Tech brand degreaser that had a high percentage of Naptha in them. Obviously from China. The same place that brought you the quality gypsum board and lead painted toys. Anyway, it works really good at removing paint. Took a good bit of it off the bottom of the car and the axle housing. So, now I use the Advance brand or Gunk for anything painted.
Anyway, what I would do is to jack the car up in the sun, remove the wheels and let it sit and get warm. Not too much of a problem in South Carolina in July. After about an hour, spray everything down with the degreaser and let sit for about 30 minutes. Then take the pressure washer connected to a hot water source and spray it all off. I've used the laundry room hot water line and the hot water heater drain valve before, depending upon my location. If you can't stand the smell of Gunk or the degreasers, then full strength 409, Purple Power, Greased Lightning or Simple Green will work almost as good. May take a couple of treatments. I use Oven cleaner on dis-assembled engine parts. Do the same thing, sit them in the sun or place in the shop oven and warm up. Spray with oven cleaner. Let sit for 15 -30 minutes, then brush/scrub with an old toothbrush or the like and then wash and dry. Nice and clean!!!
Use proper protective equipment when doing this. Filter masks with the oven cleaner and nitrile gloves. Don't forget the safety glasses as well. After this fun, don't be upset if the wife asks you to take your clothes off outside or in the laundry room.
HTH
Mike Miller