I adopted a 7 yr old yorkie (rip Brandi). Brandi would not pee or poop in front of me either, that's why you have to be even more diligent. Confinement is a must. When gets tire of being confined, he will eventually start peeing and pooping in front of you. It was even more difficult to train her because she was a rescue from a backyard breeder. She was use to pottying in her crate so confining her didn't make a huge difference. Because of that and my own lack of due diligence, it took a year to get her fully potty trained. I'm not saying it will take your yorkie that long but it does require patience and consistency.

Fortunately for me, she had no problem with going outside so that is where we started. I would say "potty" every single time she pottied. Each and every time she went outside she got a treat, a hug and a kiss. Be careful because this almost always leads to fake peeing to get a treat so you have to make sure that he is actually peeing before he gets a treat. My goal was to have her associate peeing and pooping with treats so it would be easier with training her to use the potty pads.

Like you, my schedule was erratic so I wanted to train her on using the potty pads. As a formerly confined and neglected breeding dog, I didn't not want her to be confined ever again so it was important for me to trained her using the potty pad so that she could have run of the house. (Needless to say I created a diva :p)

I purchased an expen that she couldn't escape and put potty pads in it. The expen was large enough that she would be least likely to step in it and make a mess. This is where she stayed when I was working and slept at night. When I was home, I would take her out every hour to the potty pad and place her on it and just say "potty". I showed her the treats to try and entice her. It took sometime but she gradually started picking it up. After several setbacks and a year later she was fully potty pad trained. The last 6 years of her life was spent having free run of the house with no confinements and no accidents ever again. If she had an accident it was because I accidentally closed the door to where her potty pad was. I also did periodic reinforcement by taking her to the potty pad and treating her when she pottied. This I think served as a reminder that she was peeing in the right place.

It probably took her year because I wasn't as diligent as I should have been so I don't want to scare you. There were times when I just didn't stay on top of her training (I would give her free run without keeping an eye on her). The yorkie I have now only took a few months but that was because of the lessons I learned with my first yorkie. Hope this helps and good luck. Remember patience and consistency.