Almost 20 years of sitting. The odds are really good that every system that has/had fluid in it is going to need maintenance. Gaskets and seals and hoses tend to dry out/break down over time - so even after you get the problems associated with stuff just sitting (gunk, corrosion, etc.) - you may find you have leaks you're gonna have to deal with. If it were me I'd go through each system one by one and start by changing the fluid. Brakes, coolant, engine oil, transmission and rearend. There's a good chance that anything with brake fluid in it has picked up enough moisture that you may have corrosion from the inside out. The rubber brake lines are likely completely shot. And it's almost certain that anything with fuel in it has had the gasoline break down - which usually means varnishes that LOVE to clog up, especially, small passages. And there are plenty of those associated with the F.I. system.

So, I think chasing down a possible relay problem is likely to be the least of your issues.
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82 242-6.2L; '17 Mazda3; '16 Crosstrek