Mend wires under the seat

[BPY]

Mot's coming up
Occasionally when the passenger seat is moved the service and airbag light comes up on the dash. Garage said code was the pretensioner. I'm guessing it's the wires. How do i go about fixing them? I'm guessing starting with a multimeter, but what exactly am I looking for?
Also what's the best way to access them?

Thanks and soz for being needy.

 

[BPY]

To save hassle, is the first part of this video what I need to do? ie bypass the grey thing? What's the purpose of the grey thing?

http://www.youtube.com/watch?v=f0SxaRjXhEA

 

[BPY]

Whilst inspecting the wires I found the previous owners lollipop and a pound coin!