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HelloCustomer The most common wiper failure on you mercury is the wiper control module. This is uisually the problem when you loose intermittent and/or a high or low speed. This part is only available from the dealer, and is approx. $150.00
The module has multiple circuits, or switches built into it that control the high/low and intermittent speeds. If one of these circuits goes bad, you will loose only that one function of the wipers.
As you can see from the diagram I sent you, this system does not use a relay, the control module functions as the relay. It is fed power through one 30 amp fuse, and if the fuse was blown, you would have no wipers at all.
The only other component in the wiper system is the switch, and a failure of this type is not typical of the switch. I have dealt with this problem several times and it is always the control module.