The usual place is through the hood well. Water gets in through the seal on the lid, or drips of the hood when stored wet. This leaks out of the bottom of the well into the side panels or the boot.
Many 107s are run as second cars so don't get driven much. Condensation forms in all the box sections, including the sills, and doesn't dry out. Hence rust.
Watch out for cars from the west coast (of the UK!). Mine was from Glasgow and seemed to have suffered from too many short runs in a salty atmosphere.
Get the Waxoyl out and squirt it everywhere!