The cruise module reads the voltage from the brake switch.  The outboard
lamps are regular bulbs, but the high mounted stop lamp on the hatch has
LEDs.  There is a bias voltage in the cruise module on the brake lamp
switch circuit, so if the bulbs are both burned out, the only path to
ground for the bias voltage is through the solid state lamp, thus making
significant a change in the voltage reading in the cruise module, and
confusing it to the point where it'll disable cruise.

This is one of those things I learned the hard way.  I had one of these
with cruise inop and never even thought to look at the brake bulbs.  Lots
of diagnosis and a cruise control module later, it still wasn't working.
I'll remember that one for a while.