Pull it off it's base and take it to auto parts store
It is a standard automotive relay so they will have one
What is it for?
Need to clean up the wires a bit to see their colors to make a guess but really doesn't matter at this point, replace it
My current guess is that previous owner added it for his own equipment, wiring doesn't look Factory
And by 1999 most relays were in the engine fuse box not loose in engine bay