Being the proud owner of a black on black Volvo, I can attest to the fact that ANYTHING will help. The seats are almost hot enough to scorch you after the car sits in the sun in the summer.
I'd want to turn it off in the winter, though -- that nice black leather interior is nicely warmed up on sunny winter days!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!