Ok, since you are the expert, I'll contend it's the seatbelt switch...

...where does this switch reside so I can get it replaced?

I gotta get at least ONE of these cars at 100%...

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car