According to the wiring diagram there is no measuring of resistance
differences. The test is only that the resistance of each circuit is low
enough that there is sufficient current to pull a relay.  Also, from the
same wiring diagram, it looks like the high mounted brake light is not
monitored at all.

I do not know what the threshold current is for the relays. I believe
the '90 Accord uses a dual filament bulb (#2057) for the tail / stop
light. If it were possible to jam this bulb in reversed, you would get 7
Watt stop lights and 27 Watt tail lights. This would mean that the
current seen by the monitoring circuit is only about 1/4 of what it
should be, and this may be low enough that the monitoring circuit trips
the brake bulb alarm. Check if all tail lights work, and that all brake
lights are noticeably brighter than the tail lights.

A second possible problem is a missing ground in one of the tail lights.
With tail lights off, the brake lights would work but be very dim. With
tail lights on, the tail lights would have close to normal brightness,
and stepping on the brakes would cause the tail lights to turn off and
brake lights to not work. In this condition too, the current will be
much lower than normal, and the monitor circuit may detect the situation
as a broken brake light.