This sounds odd - but put a string of bells on whatever door you use to let her out most of the time at dog nose height. My boss did this, the local trainer feels that this makes the dog associate jingling bells with going potty.
My bosses dog, now 2 years old, goes over and rings the bells whenever she has to go out, and a human knows to come open the door.
Worked like a charm for her.
Cute dogs, worth all the effort to house train.