Check out ColorPlus's website ...
http://www.colorplus.com ... they have a PDF file about leather and dying your interior. I'm in the middle of redoing my seats.
Essentially, you clean the heck out of everything, use lacquer thinner to remove as much of the old finish as possible, sand the leather surfaces lightly to open the pores, condition the leather, wipe down with laquer thinner after the conditioner's absorbed and apply the dye.
Their dye works on vinyl, too. Haven't done the dye work yet, just cleaned and conditioned. I'm not changing color, so I'm skipping the more scary steps.
Vic
88 Carrera