If you are a plumber then I am sure you are good with the hands and mechanically inclined so this should be a walk in the park for you.
One of the best ways to quickly diagnose if a problem is due to a certain component is to just replace that component with a known good replacement and see if the problem goes away. You actually have a known good replacement in the passenger-side side air bag sensor. They are exactly the same OE part number from side to side. If you were to swap the passenger side sensor with drivers and visa versa then if the sensor itself is faulty, you would no longer be getting a 22 flashcode but instead a code 32 (passenger-side side air bag sensor circuit) indicating fault had moved with the sensor and new sensor should get you back in business.
Getting to the sensors is very easy. They are located near the floor behind the lower B-pillar trim panels (this is the trim piece between between the front door and rear sliding door that extends from the carpet to about half way up )
First thing to do is to disconnect the negative battery cable for 5 minutes so that there will be no power available to the air bag system. This will prevent any accidental deployment of air bag when disconnecting sensors. (Not that it would probably happen anyway but is a good precaution).
Start by removing the front door scuff plate. Just pull straight up on it and clips will release. See first drawing below.
Now open the sliding door and remove the rear scuff plate in the same way as the front. #2 diagram.
Slide the front seat all the way forward.
Pull the front and rear sliding door weatherseal away from the area of the lower B-pillar trim.
Pull the lower B-pillar trim straight away from body to remove. #3 diagram
At the very bottom near the floor, just below the seat belt retractor is the sensor you are after. Last picture below. Electrical connector has a sliding sleeve near the sensor. Just slide the sleeve away from the sensor and connector will release. Remove the two 10mm bolts and it is out in your hand. Mark with X so it identified as the suspect faulty one.
Repeat with the passenger side, swap the two modules, reconnect the battey, and start the van, If flashcode is now 3 long flashes followed by 2 short ( flashcode 32 ), then order up a new sensor to replace the faulty one.
OE part number is LC62-57-KC0B ( that is a zero and not letter O in part number) dealer retail is $87.25 but available from
www.onlinlemazdaparts.com for $69.80.
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Pull the front scuff plate upward, then disengage clips A, pin B and hooks C from the body, and remove the front scuff plate.
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Pull the rear scuff plate upward, then disengage clips A, pin B and hooks C from the body, and remove the rear scuff plate.
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Remove the fastener.
Disengage clips A and remove the B-pillar lower trim
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Here is the side air bag sensor below the seat belt retractor.
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Could upload one more pic so here is one disconnecting the sensor connector.