l have the lock and the key, and l have the old lock out, but l cant find how the lock attaches itself to work? help,

There is a little metal rod, with a 90 degree bend in it, that goes in a hole on the lock. The locks r different for right & left doors, so do not try to use the wrong side is lock.
The lock is held firmly in place with a U - shaped metal. It is slightly bent to be springy & hold itself against the inner door sheet metal. Did the ''U'' drop down inside the hollow door?

l will try to find a photo.but no promises. You should open ur other door is panel & look at a good lock. Or go to a junkyard / breaker is & look at any peugot from a similar year.