I have installed LED lights in my toolbox in the bed of my truck what are you if the relay is waterproof and where should I install the relay?
For most part they are water proof or at least water resistant depending on brand.
Make sure your connectors or socket is also insulated and preferably heat shrink installed to keep water to minimum if placed in a wet area.
As for installation location is best choice where first protected from water, dirt, and physical damage. But you also want install placed to allow the most direct wire run path for the feed wire and control wires. I also like to keep my control wire run short and as close to the control switch device when possible but that would be based on your control device type and location.