I have an undercar wand from American Waterbroom, but they don't make 'em any more.
When I'm not using that, I just put it up on stands and crawl under with a hose and my BHBs. Actually, I don't use the wand all that much except for the first passes/last rinse in the winter. I do most of it with the hose and brushes, working on a creeper. If you do the undercarriage at every wash it never gets very bad.