Verify that it's not just the indicator lamp. Does the car hold speed if you
set the cruise control even with the dash light is not illuminated?

If it's really not setting, there is another fuse under the dash to check.
(I'm referring to my copy of the 1988 service manual so your vehicle may be
different.)  The 7.5 ampere fuse number 10 protects the main switch but
that's okay if the main switch light is on.  The 10 ampere fuse number 13
feeds the dash setting indicator.  Check that one.

Verify that the brake lights are working.

Verify that the horns work.  (I'm not kidding.  They're in the circuit too.)

There are two sets of contacts on the brake light switch.  The normally
closed set sends voltage to the cruise control unit when the brake pedal is
not depressed.  With the cruise control main switch (and ignition) on,
verify that you have 12 volts DC on both sides of that switch when the pedal
is at rest.  The wires are probably light green coming from the main switch
and grey going to the cruise control unit.  (The other pair controlling the
brake lights are white/green and green/white.)

If the brake switch and the other items listed above are working it could be
a bad ground, the slip ring behind the steering wheel, the shift position
switch (or clutch switch) or the cruise control unit.

Also make sure that you listen to Car Talk every week on National Public
Radio.