To be certain:
1. Consult your owner's manual: This is the best resource. It will have a diagram showing the location of all relays and fuses.
2. Check the fuse box diagrams: The inside of the fuse box cover often has a diagram showing the relay locations. Look for one labelled "wiper" or something similar.
3. Look near the fuse box: Even if not directly in the box, the wiper relay is usually in close proximity. It's a fairly standard square or rectangular component with multiple terminals.
If you can't find it after checking these locations, a search online for "1999 Dodge Ram [your trim level] wiper relay location" might yield helpful images or forum discussions from other owners who've tackled this. You could also take your vehicle to a mechanic or auto parts store for assistance.