I just added intermittent wipers to my 1980 Concord back in December. It's an easy plug-and-play installation, no modification of wiring or anything like that required.
All you need is an intermittent switch and the control module that goes with it. You just remove your old switch, disconnect it from the wiring harness at the big semi-circular connector, and install the new switch/module combo in its place.
When I did mine, I guess I should have slowed down a bit and taken some pictures along the way for a future "how to" tutorial. It's all back together now though, and I'm not about to take it all apart again.