Christmas markets in December in Germany.

do you use a gps (or sat nav as we call them in England) at home?

gps works the same everywhere. put in postal zone and address or name.

what advice do you need?

I've driven in Germany many times, and have always just used Google Maps for GPS. There's no need to rent one from the rental company. They may include one for free, but I just left it in the trunk of the car. It's so easy just to use Google maps like you would in the States. Germany has pretty good cell phone and satellite coverage as well, so you shouldn't have any problems with GPS.

If you're staying in Dinkelsbühl, you are relatively close to a number of nice Christmas markets. I'll be heading there myself with my sister. I would suggest going to the one in Stuttgart, and a nearby one in the small town of Esslingen, which everyone raves about. Those have good reviews. You can also visit the ones in Nuremberg, Regensburg, and Bamberg, which are all relatively close to you and easily reached by train. There is one in Regensburg that is supposed to be exceptionally good.

Here is a link that gives you locations of markets along the Romantic Road (including Dinkelsbühl's Christmas market= https://www.romanticroadgermany.com/general/christmas.php

There are so many markets - large, small, Medieval Style, more Modern, etc. Nuremberg, Augsburg, Nordlingen, etc.
https://www.romanticroadgermany.com/general/christmas.php
https://romantischestrasse.de/en/sight/romantic-christmas-market-noerdlingen/

For the large towns check out the possible parking before going because the city centers are traffic free.

Note: Obviously when driving one does not indulge in Gluhwein at the Markets.
I wonder, have you considered spending a night near one or two major markets so you can just relax and not have to drive after the visit?

For between Christmas Markets: https://www.schloesser.bayern.de/englisch/palace/index.htm