I have replaced the original interval relay for a programmable one. The interval time is programmed by switching the interval on and off so the wipers start to wipe. Then simply wait until the next "wipe" is required and switch the interval on permanently. It works so well that I seldom use any of the other continuous modes.
No rewiring required, just exchange the relay.
https://www.rendcarparts.com/volvo/p...80/RD3523610PA