"Brake pads worn" warning light question

[jdamiani]

Recently, I noticed the "brake pads worn" warning light on
my '92 100CS Quattro started coming on intermittently.  I
also noticed that when the warning light wasn't on, I could
get it to come on by honking the horn.  After replacing the
front pads (1/4 life remaining), the light still came on.
The rear pads have half the life remaining.  Any suggestions
on what else could be causing the warning light to still
come on when the engine is started?

Thanks for any ideas.

Bruce Damiani
89 100 avant 5sp
92 100CS Quattro 5sp