The interval function is timed by the Low Current Circuit Board in the SJB. There have been several reports of water getting in there and wreaking havoc on the radio, wipers etc. But if you don't have that problem, you'll need to look elsewhere.
The electromechanical relays for the washer and wiper motors are out under the hood in the Power Distribution Box. Check your owner's manual for a map of the box. If you don't have an O/M, check my signature for a link.