I remember someone telling me a long time ago that Yorkies were one of the more difficult breeds to housetrain. I don't know if this is true or not, but I could believe it! I assume you've already tried puppy pads and obviously crate training to no avail, but have you considered litter box training? Silvia has a larger-than-normal cage with a litter box in it, which she uses instead of using the floor of her crate. I know it's not the standard for crate training, but it worked best for me and it has now for four and a half years. The only time Silvia has an accident in the house is when her crate door is accidently shut or when she wants us to change her litter. It gives an alternative to pottying in the crate floor and it's easy to clean up. We use this as a permanant solution, as in Silvia never goes outside to potty. Now, if we take her outside or we go somewhere, she has no problem pottying outside, but it's not her norm. Anyway, just a thought. Here's my post on it in case you're interested:
Litter Box Training Your Yorkie