Frozen brake cable

It's really cold today, in the single digits.  This morning when I
started my '97 Camry, the brake light stayed on when I released the
parking brake.  It felt like the cable didn't release after the last
click.  It was fine last night.

I called my mechanic, and he suggested driving forwards and backwards
a little, and pulling gently on the brake cables (which are under the
car just in front of each rear wheel).  It didn't help.

Is there anything else I can try?  What about using a hair dryer to
thaw the cable?  Are there specific places where the cable or other
parts usually freeze?

I didn't think of it at the time, but it could be a different problem
with the brake system.  I'll check the reservoir fluid level. Is there
anything else?

For next time I know not to set the parking brake on level ground in
the winter.

Jimmy