Fitting an extra seat belt

Hi, we are in the process of buying a motorhome, seen one we are keen on but we would need an extra seatbelt. It has 4 travelling seats (2 rear facing, 2 forward facing) in the back but there are only seatbelts on the forward facing seats....we'd like to have one put on one of the rear facing seats. We would like a proper, certified job done and are happy to go through the rigmarole. Does anyone know who could do this sort of job? Would you have an approximate idea of price?
We are in Donegal

our last Motorhome had the same seat configuration as you describe, the forward facing seats had three point seat belts and the rear facing seats had just lap belts. so you probably just only need lap belts.