Hi Terry....welcome to the Forum
I would be tempted to drill a small hole in the rear where the water is collecting and insert a grommet once you have put some corrosion protection on the bare metal....better to let the water drain out than sitting there.
Also, worth sealing the light unit gasket to stop any further water ingress.
If it was me I would also fill the affected area with waxoyl or similar anti-corrosion wax treatment.
Good to have you onboard