Couldn't care about that "Bitching Betty" warning system as it does not function for anything other than the doors, low fuel level, parking brake, key in ignition and lights. Otherwise mention of this system is a non stock part and outside of the scope of this discussion.
Now to get this discussion back on track again.
As far as the module on the back of the cluster both the ND and NS versions both have just a diode box for the carb'd models and the same board used for the PGMFI for signal amplification. Location of the ND plug is horizontal and on the bottom vs the Seiki type is vertical and of to the side. The picture you displayed is most likely a Seiki type from a non PGMFI car and from the part # the two beginning letters are NS . You might want to double check that speedo, factory manual spec is 4 pulses per 1 revolution.
Now on the reed switch, they are pretty robust and really the two main sources of failure for those kinda devices is a cracked capsule allowing air and moisture to alter the contact properties and open/close characteristics. The other is using too high of a potential and current that ends up producing unwanted arching. With that being said if used properly they can have been known to have decent lifetime cycles into the billions depending on design. Without any additional circuitry, using a simple power supply,resistor, reed switch and magnet you can obtain a waveform similar to this ( note additional components could be used like capacitors and zener diodes to clean signal up more):
I'd like to see that factory schematic, as far as I can see the only wires to the speed pulser is the one wire in from the cruise control unit and a ground wire. The speed sensor amplifier only have a power wire that is fed off of a 15A fuse, a ground wire and a output wire to the PGMFI / solenoid control unit. Schematic found on 25-50
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