The invention relates a connection unit comprising an insert provided a front part and a back part provided with longitudinal through holes. At least one longitudinal through hole in the front part of the insert coincides with a longitudinal through hole in the back part. The connection unit includes at least one contact extending through a longitudinal through hole in the front part and through a longitudinal through hole in the back part of the insert. A front portion of the contact is provided with a stop pressing on a stop in the longitudinal hole in the front part of the insert. A back portion of said contact is provided with retaining means which require said back portion be forced into the longitudinal hole in the back part of the insert. The invention also relates a mounting process of such a connection unit, as well as a connection kit.