Hi, not sure about Doxies being known for having bad breath, but there may be a possibility that she is over producing a certain bacteria in her mouth which is causing it???.... a friend's dog suffered from this, but it was easily treated with a course of antibiotics...or there could possibly be decay somewhere that you can't see.
How do you try to brush your dog's teeth?? You don't actually have to physically brush them, just rubbing the toothpaste over their gums and teeth works just as well. I started cleaning my dog's teeth from the 1st day I had them...poor things

The easiest way to get them used to it is to start by just letting them lick the paste of your finger and progress onto rubbing it on their gums etc after a few days have passed. Best of luck