The lawn sprinkler suggestion is a good one; if you attach a broom handle to a board and mount the sprinkler to it, you can more easily position it under the vehicle.
I hate to drive even my old beaters in the PA salt. Some other ideas I have used to keep the corrosion at bay:
I hose off the vehicle several times a week, more often if roads are wet and salty. Most important is the undercarriage, especially the nooks that will harbor salt and slush.
If you jack the car up with a floor jack, those areas are much more accessible. Lying on a creeper helps too, and keeps you out of the water on the garage floor. If you wear a headlamp (available at any K-mart, etc,) you can see and use your hands as well. The long watering wands used for gardening have a variable spray head that you can position and also let you choose an appropriate spray pattern. They are long enough that you can reach well under the car if you don't want to jack it up.
You can adapt one of those hose-end fertilizer sprayers such as the Miracle-gro brand to mix in some car wash detergent; I used some heater hose and some hose repair fittings on the inlet and outlet sides, along with some quick-connect hose fittings so that I can fill the fertilizer cup with soap, snap the unit on to my hose, and then snap a variable-spray nozzle on to the fertilizer sprayer. The unit quickly snaps off the hose for rinsing with plain water. I think this is more effective than water alone, and the fertilizer cup hold enough soap to last at least through one wash. Wal-mart sells a big jug of blue car wash liquid rather cheaply and for what it's worth, it even has wax in it.
Mixing in a little hot water makes the job slightly less miserable in freezing weather, and makes hoses more cooperative.
Buy the best quality hose that you can afford to avoid aggravating kinks.
Use those quick-connect hose fittings if you use several hoses, nozzles, sprinklers, etc; they work well.
When you are done, be sure to open the doors and wipe dry the weatherstripping in freezing weather lest your doors freeze shut overnight. Don't ask how I know this. I give the weatherstripping a spray of silicone occasionally as well.
I am sure my neighbors have, while passing by of a brutally frigid evening, chortled on seeing me in frozen clothing and weirdly flashing headlamp desperately trying to rinse off soap suds before they froze to my car, but then there are few "88 Voyager minivans with 210,000 miles on them whose undercarriages are as nice as mine!