I rescued an older yorkie as well and potty training her took some time but with patience and consistency, it can be done. To trained her for outside I used treats. Everytime she would potty, I would say potty while she was pottying and then reward her with a treat after she has potty. She learned to associate the command potty with pottying and getting treats. Now the drawback to the treat method is these little guys are so smart that Brandi would start fake pottying to just to get the treat, so you have to make sure your little one has actually pottied before getting the treat.
Brandi is potty pad trained (great for late night potty breaks and cold winter days) and goes outside as well. To trained her to use potty pads, I simply put the potty pad where she had most of the accidents and she gradually started using the potty pads. I would also place her on the potty pads and say "potty". If you don't like the location of the potty pad, you can always gradually move it a little bit at a time to the location you want it to be. She now has full run of the house and if she has an accident it's it's because I didn't change the potty pad quick enough for her.