Hover over a wire to confirm the colour
How to wire it for a two-speed motor: With the standard wiring the motor when parked is putting out a earth on the brown/light-green which goes through the standard switch in the off position and back to the motor low-speed winding on the red/light-green. This earth must be disconnected before 12v can be connected to the low-speed winding to start a single sweep or you will blow the green-circuit fuse. Remove the brown/light-green from the existing switch and connect it to the 'normally closed' contact of the new switch or relay, which needs to be of the single-pole, double-throw (SPDT) type. The 'common' contact of the new switch or relay goes to the existing switch where the brown/light-green was removed from, in the above diagram with a yellow-light-green. The green circuit is extended to the 'normally open' contact of the new switch or relay.
If you can't get a SPDT non-latching switch you can wire a green to a normally open (SPST) push-button which will operate a relay with the SPDT contacts when the button is pushed, which then controls the wipers.
On cars with the wiper switch on the dash the changes can be made with extender wires and connectors making removal simple and the cars standard wiring is not altered in any way. If you have the wiper control on the column stalk you will need to cut the red/light green and splice two new wires in to the 'common' and the 'normally open' contacts of the new switch or relay. Make sure you get them round the right way, incorrect wiring will blow the green fuse.
How it works: The new contact effectively disconnects the earth from the motor park switch whenever the main switch is off and the wipers are parked, and replaces it with 12v to start the motor. As soon as the wipers move from the parked position the earth from the park-switch is replaced by 12v, which means that when the flick-wipe/intermittent contact is released, if the wipers are still away from the park position, they continue to run until they have completed the normal parking sequence. If you hold the flick-wipe switch down the wipers run continuously until you release it, after which they will park normally.
You could interrupt the red/light-green slow-speed wire from the manual switch to the motor and disconnect the park switch earth that way, but the beauty of the above when used with an intermittent wiper control with it's own switch, you can immediately override the intermittent circuit by turning on the main wiper switch. If the red/light-green had been interrupted, unless you turn the intermittent circuit off as well as turn the main switch on, every time the intermittent circuit triggers it will cause the wipers to stutter on the screen. (Note this does not apply to the Smartscreen intermittent wiper control as the main wiper switch is used to control that anyway).
Note that the story with the Mk1 GT is very confused. It could have the same system as the Mk1 roadster, a 2-speed motor using the same park system as the single-speed, a single-speed motor with the later parking system, or even the same motor and park system as the Mk2 cars. Whilst the third variation - single-speed motor with the later parking system could in theory use the above circuit, the polarities in the circuit may be reversed i.e. the park switch puts out an earth to run the wipers back to the parked position, then stops the motor with 12v. In that case instead of 12v being connected to the normally-open contact of the SPDT switch or relay, it must be an earth or the motor will not run. You will need to identify the park wire, and test it while the wipers are parking to see if it starts off with 12v and finishes with an earth as in the above circuit, or the other way round.
Finally if you have bought an after-market intermittent wipe system, study its instructions and diagrams carefully. Not all of them may be suitable for each type of motor and park system.