Mike,
I have read some of your post and you seem to have a very good handle on
electrical systems.  Maybe you would be willing to take a shot at my
problem.  As stated earlier I have a 91 Olds 88 3.8 liter.

Upon starting the vehicle as it runs through the "service engine light and
air bags, seat-belt, etc" dash lights my directional arrows and high beam
icon light-up for a moment and when the start-up lights on he dash clear the
directional and high beam icon go out.  Now that is just the beginning.
After that everything works fine until you turn on the head lights, upon
doing so the speedometer stops working (shorts-out) the directional and high
beam icon stays on, (if you turn the headlight switch off the lights goes
off and the speedo works) the head lights work for a about 3 - 4 min, then
they begin to start flashing, tail lights and parking lamps are not
affected.  The high beam and low beam work, however the high beam light
works in reverse, when the light is on, you are on low beam and vise versa,
after several minutes of flashing the stop working all together.

I have not replaced the headlamps since we owned the car, (ten years).  I
took the headlight switch apart and did not see any fuses, diodes etc, just
copper contacts, its is a rocker type switch.  I bypassed the switch and
same problem.  Our local tech college auto teachers thought is was the
instrument cluster, so I remove that and the you don't need it in place for
the headlights to work and the problem was still there.

I am do not know what else to try,  I don't know if there is some type of
relay somewhere that might be have a short in it.  To me it seems like a low
voltage short, if you play with the dimmer switch, which is part of the
headlight switch and is just a dial type you can get the speedo to work
interminably but does nothing for the headlights.

Any help would be appreciated.

In a un-related problem which just started, ( by the way I have had this
problem for about 6 months) I had gotten a service engine light so I checked
the code which can up 41; cam shaft sensor, so I replaced that today and
still have the same light and code, cleared the ECM twice, checked voltage
at two of the three wire to the sensor and had 10 volts, which is what is
should be.  The parts store thought is was a defective sensor and
unfortunately is out of stock, need to wait until next week for a
replacement to see if that is all it is.  Just thought I would throw that
out there.

Thank you,
Brian