I took apart my light stalk last year when the contact 'unsoldered' itself (I have an '88 328GTB). Evidently this is a fairly common problem, but easy to fix. You can use your fingernail or a razor blade to slide the inner 'ring' away (towards the steering column). Work a little at a time around the 'ring' - it should slide apart with just a little pressure toward the steering column. Once this is moved apart, the plastic end piece (knob?) can be pulled off easily. Just make sure to notice the orientation for the end piece so you can (re)attach it when you need to. Otherwise the writing on the knob might be upside down when finished. I don't recall exactly but I remember looking inside of it and it appeared it could only go back on two ways (by rotating it 180 degrees). Hopefully the attached pics will help.
If you need to (re)solder the wire just clean the contact area (600grit sandpaper works great) and gently resolder it without putting much pressure on the stalk. Also, be careful with the soldering iron as it gets hot and can drop solder on your leg (don't ask!). A towel or something else protecting your carpet (and leg) helps!