Hi Buzzle
Excellent advice from everyone
I also have Chihuahuas and they can hold on all night from the age of around 4- 6 months. I have a 15 week old pup and he can, and does ,hold on all night no problem.He is house trained already.They sleep in crates overnight (only) and this teaches them to hold on as they do not want to pee/ poo in their bed.This is very effective. My Chis can spend all day happily outside in the winter when I am not home. ( I live in Victoria) they have shelter to protect them from rain and wind and warm beds. So your Chi should be fine outside even if she/he has a nice warm box.JMO but I would lose the pee pad and loo and start afresh by taking her/ him outside every half hour with a command ,just the command you normally use, and when she goes give her a treat like a very small piece of schmacko (they love them)and make a real fuss over her with praise and clapping and hooray pee outside,but dont give her the treat for anything else but pee/poo.This really works for me and it doesn't take long hope it works for you
Good Luck