For a D1 as the D2's are bonded cut the rubber seal and remove the glass and then remove the remains of the seal, you may find rust underneath the seal so that area must be made good, fit a new seal and either refit the glass yourself of get a specailist to do it.
If you are just going to use sealent then silicone will just peel off you need to use either Sikaflex or the stuff the windscreen fitters use.