The seat belt retractors have an explosive device that causes the seatbelts to snap tight on the wearer in the event of an accident. These are ignited via the wires that are connected to the belt units by the airbag control module.
Just one of the many warnings from the dealer workshop manual...
"Before handling pyrotechnic components of the restraint system,
for example before separating an electrical connection,
the mechanic must electrostatically discharge himself/herself.
Electrostatic discharge is achieved by touching earthed metallic
parts, such as the door striker plate."
Don't be frightened by the above, just treat and handle these components gently and with respect! If in doubt have the work done by an experienced person.