To be absolutely certain and avoid unnecessary disassembly, I strongly recommend:
1. Consulting a repair manual specific to your 1998 Dodge Ram 1500. A Haynes or Chilton manual will have diagrams and instructions showing the exact location.
2. Using a parts diagram for your specific vehicle. Most auto parts store websites (like AutoZone, Advance Auto Parts, etc.) allow you to enter your vehicle's details to view diagrams showing the location of parts.
Simply searching "1998 Dodge Ram 1500 heater control valve location" on image search engines like Google Images might also reveal helpful pictures from others who've tackled this repair. Remember, though, that these pictures might not be universally applicable to every model within that year's range.