Who is responsible for certification?

We are an upfitter, and one of our customers is a large municipality that operates and maintains a fleet of commercial trucks. They often buy new incomplete chassis-cabs from a local dealer to swap underneath a used truck body and/or equipment which we originally built for them. They are now asking us to certify their bodies, and we are not sure if we should. Who is responsible for certifying vehicles that start as incomplete chassis and have a body added from a used vehicle?