'87 5KTQ Instrument Lights - ON Equals OFF?

[M Connors <mconnor@frontiernet.net>]

I have recently developed an annoying problem that is experienced during
nightitme driving.  The problem is when I go to turn on either the
headlight or parking lights on the car my dash lights go out
completely.  Even the climate control head temperature reading
disappears.
Needless to say, I am finding that Black on Black gauges viewed in
compete darkness just isn't cutting it here.  Perhaps, I have
unknowingly activated some type of innovative nighttime speedo cloaking
feature whose description has been mysteriously omitted from my owner's
manual...
In any event, I have checked both fuses #12 and #14 to no avail.(Both
good)  It was interesting to note after a "reseat" on the fuses my dash
lights worked with my headlights for a day. Now, I am back to my
original problem again.  Does anyone know if there are any relays or
other points of failure associated with the instrument light cluster
that I have overlooked?  It seems as though I have stumbled on to a clue
when I reseated fuses #12 and #14 which fixed this problem temporarily.
Any BTDT as to what to look for would be greatly appreciated. Thanks in
advance.

Regards
Mark