The seat back cable can be purchased from www.volvoparts.com. But,
you will ahve to call -- they won't have it listed on their web site.
There are many sites that sell parts and maybe someone knows one
that has more parts listed on thier web pages.

Instructions to replace the seat back cable are at:

http://www.volvospeed.com/Repair/seatcable1.htm

I believe the headlight wiper only moves when you squirt water
on the windshield (pull back the stalk). But, the wipers do
break and the windshield "squirter thing" on the hood can plug up
(actually, the check-valves under the hood's insulation plug up).

I'm not sure about the exhaust.