The information you have is correct. The POSITIVE battery terminal connects to it. Just follow the battery cable and you will find it.
There are two different 4.0L V-6 engines used in Ford Explorers in the 1998 model year.
One is a Dual Overhead Cam (DOHC) engine. (VIN code "U")
The other is and Overhead Valve engine (VIN code "X")
What "VIN code" means is that the 8th digit of your VIN will be either a "U" or an "X". The 8th digit in the VIN identifies the engine that the vehicle is equipped with. You may want to use this information anytime you are talking about your truck or trying to get information or parts. These two engines are COMPLETELY different, so it can save you a lot of frustration if you give the correct information. Otherwise someone may just "assume" it is one or the other and you will get incorrect information or wrong parts.
Anyway, the actual location of the starter relay is the same for BOTH engines. The only difference is the type of cable hook-up that is on them. (see pictures below)