I've done a lot of research on this topic. Here is what I know:
Switches:
1. The 4 wire switch does only support low and high speed. The water is activated through a valve. Wipers will not work when the water valve is activated. The connector has 5 terminals but one is not populated.
2. The 5 wire switch adds the timed delay relay option and wipers on when water valve is open. Note that you can't run an electric water pump with this switch. It just triggers the delay relay to operate the wipers when the valve opens.
3. There is a 6 wire switch used in Super Beetle that has an additional wire for the electric pump. This wire doesn't go to the connector and has it's own spade connector that should be routed to the pump (see picture - note that this switch has both the valve and the pump wire so you can use electric or pneumatic washer system)
4. There is an old 924 switch that has the same functionality as (3) but has a larger connector with 7 terminals. It's an extension of the 914 connector with 2 additional terminals, one of them empty. The pump wire is in the extension. (see diagram below)
5. The signal switch has a tab that prevents the wiper switch to go into the intermittent position. That tab must be removed when an extended switch is used.
Harness:
The 914 harness had the option for intermittent wipers. In some models, everything is in place and an intermittent wiper is installed in the front trunk. If that relay is not there, the wire from the switch (5th terminal) is probably still in place but it's wrapped in the harness to prevent a short.
You can add the relay but you'll have to cut the return power from the wiper motor and plug the relay there, together with the said wire and power and ground.
My opinion, and this is what I've done to my car as well, is that with all the trouble you go through to add the wiper relay, it's better to add a programmable relay that will permit you to control the wiper interval. I use a VW relay that allows that just by switching the wipers on-off-on once to program the interval. The relay wiring is a bit different but you need to add that relay anyway.
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