Neither cooling fan coming on

Neither of the 2 cooling fan come on my 2000 Deville, even with AC on high.
I've seen this same issue posted on here several times with the Deville's and
Seville's Northstar engine but I have never seen a follow-up post as to the
cause and fix.

I previously had  2 different mechanics working on an overheating issue.  In
total, they replaced the radiator, water pump, thermostat, hoses, and coolant
cap (so now I'm wondering if one of the mechanics bumped or screwed
something up or whether radiator fluid shorted something else out as a
consequence a while ago). I know the fan used to work prior to these
replacements, as I noticed the fans working when the vehicle 1st overheated
and I left the fans on to cool the engine - the vehicle was immediately towed
to the dealer (who said it would cost gobs of $$$ and not worth it for an old
car). So it was towed to a local mechanics (husband of my wife's friend), who
couldn't fix the problem. So it was eventually moved to my "trusted"
mechanic. After being "repaired", the vehicle overheated slightly on the way
home and I was told there may still be air in the lines that still needs to be
purged out of the coolant line. My wife had enough and we subsequently
bought a replacement vehicle and put the Deville on ice. Now I'm trying to
revive the beast and have fixed almost every other issue, including new
issues that arose from sitting, including purging the air out of the coolant
system, which is when I noticed that neither cooling fan now runs. So I
checked fuses and they were good. I tried to use a multi-meter to test out the
outlet holes for the relays to check for power. However, the outlet holes were
too narrow for my leads. Then I direct wired the fans and got driver's side to
come on. I was unable to get the passenger's side fan to come on, being
direct wired, but I couldn't really reach very well so doubt I had leads in
correct spot. So I was thinking relays and swapped out the 3 fan relays and
still nothing. I'm getting the sense that it may be a wiring harness issue
somewhere, which is sort of above my pay grade. I also haven't been able to
find in my research which of the fans comes on for: (1) engine start up, (2)
for AC turned on, and (3) when engine temp reaches a certain level.
Thoughts? or anyone actually determine their own fix to this issue? I may go
back and try to hard wire the passenger side fan again because I would feel
a bit better if I knew for sure both fans actually worked - but I'm at a loss as
to what the issue could be.