It's on one side of the intake manifold, I'm not sure which for the 98. It looks like this on 3rd gens:

http://i10.ebayimg.com/02/i/03/c0/1d/c9_1_b.JPG

It's usually not the valve that goes bad, but the EGR ports that get clogged. I think on the 98 you can just unbolt a cover to access the ports, but I have only done it on my 92 where I had to drill and pull the port plugs out, then clean out the ports.