My 68' Camaro has a similar problem.  I cleaned everything with a
chlorine solution and it was good for a while.  The mold came back
after a though.  Now I'm resigned to just replace my houndstooth seat
covers and buns.  My seats are part cloth and part vinyl and I don't
see any other way to get rid of the mold.  It's a good thing I'm just
putting my car back together or I'd be pretty upset about the extra
cash I have to lay out for the replacement Interior pieces.

                  ...Ron
--
68'RS Camaro
88'Formula
00'GT Mustang

If I was faced with having to replace seat upholstery, I would remove the
fabric and run them through your clothes washer with reg laundry detergent.
I have cleaned seats this way and they have come out fabulously.  This won't
deal with the foam and in all probability the mould spores are living in the
foam as well.  If they're not too expensive you could just replace them.
Alternately you could soak them in some kind of tub with a diluted bleach
solution for a while them rinse them several times in clear water to ensure
you get all the bleach out so that it doesn't leach into the upholstery or
damage the foam.  Bleach will definately kill the mould but it may also
damage the foam, but it may be a worthwhile risk.
Steve g.