Duct Tape n Bailing Wire

1999 Tahoe Old style body 2 door 5.7L intermittent won't start or dies within 1 or 2 seconds. When it does start it runs just fine and doesn't die out while driving. Recently changed out multi-function switch to resolve no lights loose burned wire issue (removed lock cylinder in the process/then re-installed same lock cylinder). I have the "passlock" type key (no FAB; no chip; no resistor). (Instead it has Hall sensor/magnets in the cylinder/housing). After reading other folk's postings on internet I think (but not positive) that my hoe doesn't have a BCM. I've checked all over under the dash [no console-just a hump].

Since the multi-function switch change out the "security light" on instrument cluster alternates: sometimes it goes out after the initial few seconds upon start up/start up try; other times it comes on when I'm driving and stays on solid, either until I shut off the hoe or just on its own while I drive. If I've been driving it & the light stays on solid, if I shut the vehicle off then sometimes the vehicle starts the next time I try to start it; sometimes it doesn't or dies after a second.

(Even though this does not have FAB, chip, resistor I still have took a shot in the dark and tried the relearn/reset procedures: (passlock reset procedure--The BCM relearn [in case I'm wrong about there being no BCM] 10 minute [times 3 since a 'truck']...no results except the security light did go out after 11 minutes.
I've tried the "remove positive battery cable, touch nothing else, touch the positive poll 3 times, wait a few seconds, reconnect good. 80 percent of the time this has allowed it to actually start and stay running until intermittent times it will or won't start on the next tries. But...I am not sure if this is what has allowed it to start "sometimes" or not--because after about 10 minutes on the phone with tow company it has started when I've went back out [canceled the tow].
That's most of the "background" I can think of. Question: Does anyone know how my passlock "communicates" with my VCM/PCM since I don't appear to have a BCM [Body Control Module]? I figure the IPC [Instrument Panel Cluster/Control] gets the signal because the security light lights. (Maybe it's not getting the correct amount of volts?)

Does anyone know where all I could check and for what [with a multi-meter], and in what order, since I don't have a BCM most likely? Would the ignition cylinder alternate between sending the correct amount of volts and then other times not send the correct amount of volts if it were going out? [The hoe didn't have this no start problem until the multi-function switch change out]. What I lack in skills, understanding/know how may be made up in perseverance. Thank you for any ideas.