Cab rear seat belt receivers are different to a coupe as well.

Easiest thing to do is remove the rear seat and remove the bracket and install a full set of sedan belts, which screw into the holes where the bracket was installed.

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David Hendy
'97 Renntech E60RS
'97 GMC 2500 Sub' Diesel
'95 E300D
'88 300TE
'88 250TD
'84 L/Rover 109