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This video might also help:
The guy is good at explaining engineering stuff to people with a good dose of humour and a potty mouth.
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airbag warning light
A common issue with Citroen/Peugeot front seats is if they get pushed/slammed back when adjusting it does dislodge the wiring plug enough to show a fault. However I think all your problems are down to a flat battery. A new/good 12 volt battery has getting on for 13 volts in it so your <10 volts won't cut it. Where I work it's not unusual to have to piggyback two jump packs to start some cars.
May be worth clearing out underneath, disconnecting/reconnecting everything. Try everything before I submit to getting completely rinsed!