the pre tensioners are the seat buckles, they are mounted into the base of the seat frame. usually there are 2, one has the seat buckle on the other has the outer seat belt cliped into it.
you need to match up the ones on your old seat with a replacement. some do have x2, some have x1 and a dummy one.
once you disconnect the battery then take the seat off, bolts/nuts are under the car, 16mm deep needed for inner ones (hidden in boxmetalwork,behind rubber grommets)
once reconnected some lights will go out, but you still need access to a can-clip or to take the car to a garage/dealership for them to clear and test all airbags and check there are no extra errors.
can-clip units are available online (
[i got mine from a UK stockist, so delivery was in about 3 days]
you need a laptop with a 1024x768 screen to use it.
the only drawback with it is you cannot get the diagnostics hardware to physically test the complete airbag system. you can get resistance readings from all the airbags, but doing a proper tests cannot be done. (i havnt done a airbag unlock/reset/clear, so am unsure about doing it)
to do that you really need a proper garage with a full official can-clip diags system.
you local garage can clear all the errors, if the cars already there.
the steering wheel airbag needs testing! just to make sure it failed to go off.
(a missfire on an airbag could mean it will go off later at random!!! so it needs to be checked)
once all the repairs are done you need to get the car checked for geometry on front/back for camber/toe. any specs out of factory settings can be adjusted with stainles steel shims. (price should be under £150 for adjutment, basic check should be about £50)
dont forget to replace the bent rim and tyre.