The Carreras and SCs have two pressure actuated switches mounted on the
master cylinder. There is no adjustment.

If the lights work, but require excessive pedal pressure to turn them on,
then there are a few possibilities. One is that both switches are failing
and should be replaced before they stop working altogether. Another is that
the brake fluid is old, has accumulated moisture, created "gunk" in the
master cylinder and switches, thereby requiring greater pressure. And then
there's the possibility that your pedal is requiring greater brake pressure
due to failure or reduced effectiveness of your brake power assist unit. If
the car is stopping without excessive pedal pressure, then you can probably
rule out the power assist issue. That leaves either the switches themselves
or the gunky fluid. In either case, the switches have to come out. Once
removed, you can inspect for the gunk. I'd plan on replacing the switches,
in any case, and if the fluid is suspect, you can try flushing the system
with new fluid. If that doesn't do it, it's probably time to replace the
master.