GM, Bit surprised it is not already there, as my 1999 car had it as standard.
If fitted, it should be on the bulkhead under the dash, by the wiper motor on the left-hand side. If not fitted, and if you are lucky, there is a connector in about the same place, which if you pull it apart, features 4 pins in one side and 5 in the other. This is where the intermittent wipe box fits into the wiper circuitry.
If you haven't got that connector, or the box, then it gets a little more fiddly, but nothing insurmountable, I'm sure.
N.