Dog-sitting chihuahua X, it keeps going to the toilet inside?

Do you have pet kennels for them? Sounds like they need to sleep in a confined area in a kennel. If you do not have one, they can be rented for not too much or may find one at a thrift store. Keep them in another part of the house at night so you don't hear them crying or whining. They will settle down. Make sure you don't feed them after 7pm and pick up the water bowl at night. Stay outside with them before bedtime until they go and reward them with a treat. if the garage is not too cold, maybe you could set up an area out there for them. They are confused and emotionally going thru separation anxiety probably.