2 year old toy fox terrior/housebreaking

Question
I have sort of inherited my sister inlaw's toy fox terrier, his name is Ricki, he is almost 2 years old.  The dog was at Petland for the first 4 months of his life with no care or training, I understand that a TFT needs that attention asap.
My sister inlaw has twin 6 year old boys, so the dog has never had any kind of training.
   We are having problems housebreaking him.  I started to train him the same way I trained my german shepard/black lab mix, and I have found out that approach doesn't work for dogs like Ricki. We also have a chihuahua/dachshund(Tinkerbell) who is housebroken and a great dog. She doesn't get caged at all, and she sleeps with us at night. Since we got Ricki, we treated him the same way we treat Tinkerbell.
 My question is that how do I housebreak a dog that is afraid of me all day except at night during bed time. We are  gone at work all day long, I work M-F 7a-4p, my wife is a nurse and she works form 7a-7p 3 days a week, but different days everyweek. When we get home there are accidents all over the house, I let him outside and he will not come back in until he cannot see me and my wife opens the door. I need  some new advice. Please advise. Thanks

Answer
If the dog came from Petland, I guarantee you he was the product of a puppymill. I do not know how knowledgeable you are about puppymills, but basically, a puppymill is like a production line, where dogs are (usually) kept in cages for their entire life, and bred as many times as is physically possible for profit. Puppymill breeders have little to no regard for health, temperament, or conformation. Puppymill breeders are in it for the money; they do not care where their puppies go, and that is why they sell them to pet stores. That, and they don't want people coming to their facility and seeing the conditions the dogs are kept in, and just how many of the poor creatures are under their 'care.' I tell everyone never to buy a puppy from a pet store, no matter >>what<< conditions the puppy is being kept in, or how cute the puppy is, or how sorry they may feel for the puppy. The reason why is because every time someone purchases a puppy from a pet store, they are filling the pockets of the puppymill breeders, and encouraging them to continue what they are doing.

If you have not already done so, you need to take a look at http://housetraining.angelfire.com I have compiled all my suggestions re: housebreaking there. The one main thing you are doing wrong is giving him free access to the entire house when you cannot watch him, and letting him outside off lead. He needs some boundaries.