First thing I'd check is the wiring under the drivers seat and clean the connections. Remember to disconnect the battery well in advance of fiddling with any airbag related wiring.
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Do you get the passenger airbag deactivated light on as well? Even if the van doesn't have a passenger airbag I believe this is one way the multiplexed wiring used on Citroens communicates a fault with the comms systems. I had a broken wire in the pair of wires that connects two of the many computers. Both airbag lights were lit. I suspected seat wiring and removed connectors and soldered connections (didn't have lexia at the time). Wasted my time as connectors were not to blame. So you need to get Lexia diagnostic to pin point. Good luck.