front seat belt mechanism

i need some infomation please about the mechanism for the front seat belt, drivers side.

i got in the car the other day and buckled up, heard a noise from behind the seat and the tension in the seat belt had gone.

the seat belt now does not retract and pulls all the way out. i was wondering how it works and what id need to replace to repair it. is it eaiser to buy a new unit?

im hoping its just a simple spring or somethings as we have plenty of them at work!

its an 83 gti if that helps at all

cheers cav