Marilyn- I've heard that if you book through Enterprise in the States, it's much cheaper than booking through them in Germany. We tried it for our trip next month to Ireland, but it didn't work for us since we have a Germany address. Look up their number on their website and get a quote.
For the route, www.mapquest.de is great for maps (www.map24.com as a back-up), make sure you spell out the full name of Rothenburg ob der Tauber. There are multiple Rothenburgs, Rothenbergs, Rottenbergs... You can change to English on the bottom. It's only about a two hour trip (compared to the 3 1/2 hour train ride). Make sure you pay attention to which town that Mapquest tells to go towards, and NOT the direction N, S, E, or W. You will find no such directions on any signs in Germany, just the next major town you are heading towards. Other than that, signs are extremely easy to follow and you should have no problem. This is coming from an extremely directionally challenged woman!