O.K., finally figured out what was going on with my intermittent no low beam problem. Well, I knew the problem was likely in the headlight logic module being a bit blinky (no pun intended), so I found another used one I had stored away for a “rainy day” and swapped them out. That solved that little problem. I also corrected a goof of mine with the headlight bulbs, in that the last time I replaced the pair of bulbs I somehow put the retaining rings back on the lights FIRST, and then stuck the new bulbs in their holes from BEHIND them, instead of the other way around (and yes, that’s possible to do, sad to say ).
Anyway, after doing all this, now for some reason I’m having a bulb failure light on. I’ve double-checked and none of the bulbs in the car in the BFL circuit are out, so what gives? Since the headlights are not included on that circuit, I don’t see how doing the above could have triggered anything to change in BFL-ville (i.e. nothing changed the resistance of the circuit).
If this is the five spade twin bulb failure module, then it works as follows:-
The power comes in at “B” (for battery) and then tees to both “L” (for lamp) connections. Before it gets to “L”, each branch goes through a reed switch coil.
The reed switches are arranged in series and ground a resistor from “B”, so current will flow through the resistor and keep a transistor’s base grounded if there is current flowing in both actual bulb circuit connected to “L”. This means the light stays off.
When there is insufficient current flowing through the “L” circuits but 12v is still coming in from “B”, the reed switches are open circuit and the current through the resistor finds its path to ground via the transistor, which switches on the warning bulb in the dash.
Your warning bulb can only light if you supply 12v to the bulb failure unit AND either (or both) “L” circuit(s) do not draw enough current to make the reed switch(es) close.
This means you have to power the lights via the bulb failure module’s 12v supply and the bulbs have to draw sufficient current to have the magnetic effect of closing the reed switch.
You have changed the bulbs, so the new ones either don’t draw enough current now when on, or, when turned off, there is still a path to supply 12v to the unit, but not one which draws enough to close the reed switches.
You may see corrosion on the spade connectors of the unit which affects its operation.
O.K., the cause finally revealed itself … Looks like when my tech bud reinstalled the headlight bulbs in the correct configuration he somehow tugged or bumped the wire going to the parking light inside the passenger-side headlight assembly. I guess it was just loose enough that the parking light still illuminated when I went to visually check it but yet the resistance in the circuit was increased enough to trigger the B.F.I. light. Eventually the wire came loose completely (probably when I hit a bump), falling inside the compartment and turning off the light. Plugged the wire back in and everything is normal. (thanks for the detailed info on the system, though - good to know for future reference).
btw, very interesting note - by placing the headlight bulbs inside the retaining rings as they should be instead of behind the rings like I had them that slight forward moving in position seems to cause the lights to now aim a bit more upwards. The light beams are now pointing out correctly ahead of the vehicle instead of dipped a bit downward.
I also corrected a goof of mine with the headlight bulbs, in that the last time I replaced the pair of bulbs I somehow put the retaining rings back on the lights FIRST, and then stuck the new bulbs in their holes from BEHIND them, instead of the other way around (and yes, that’s possible to do, sad to say 
). 
(thanks for the detailed info on the system, though - good to know for future reference).