As a person that deals with fuses blowing because of heaters (never have had
the "pleasure" - more like torture i'd bet, or working on automobile seat
heater systems) quite often... it's almost never the fuses fault it blew...
TRY changing the fuses if they are bad, but they will more then likely blow
again, and do not install a higher rated fuse (or try the old
penny/paperclip trick... all this does is cause your wiring to turn into a
little seat heater of its own).

Heaters eventually burn out, but it's usually the control circuit,
especially when full current is flowing thru it... Don't know how Volvo did
the control system, as there is a temperature sensor which causes power to
be reduced after a while to provide comfortable heat, not so you burn your
bottom. Could be the switches for it too... that would be simple enough.
Could be a bad wiring harness.

I'll put it to you this way... If the seat heater break on my car - despite
my electrical knowledge I think I might *might* live without them... I hate
it when anything doesn't work, but it might just be too much
time/effort/money to be worth while... especially on an older car (like our
93 960... tho on my 99 VW Diesel i'd get them fixed, in the winter the cabin
heater really sucks and you need the seat heat - Volvo's on the other hand
seem to have good heat and it comes on fast).