There is a relay for the wipers so that can be your problem. According to oldskool73 in this thread, he has a 97 so you should have it for your 98 as well. Factory wiper relay is 98661510300. It is pretty easy to locate and the relay is located above the fuse box underneath the driver side.
I changed it to a VW/Skoda one to get the intermittent feature last year based on his suggestion. It works and I'm happy. According to some with the intermittent wiper dial, this mod works better.
Adding variable intermittent wipers on an early box, the easy way