I have built and fitted this kit. When the engine is at tickover it works perfectly well. However when in motion things go wrong! The wipers will sweep 2 or 3 times, then pause, and so on, rather than sweeping once and then pausing. Also all the time the device is switched on, on any of the delay settings, the relays are constantly “chattering”.
The specs say the input voltage is 12v to 15v, but I suspect that voltages higher than 12v are tripping the device. Someone has suggested to me that it may be voltage spikes tripping it instead. Their idea to cure this is to place a 220uf 22v electrolytic capacitor across the input terminals of the device, to smooth the power supply.
From your description it sounds as if you are talking about the K2599.
If this is true does the unit do this with the engine off?
If not you may want to check your alternator.
This may be cause from one of the diodes in the alternator going bad or gone bad.
I also have this problem and my car has a generator not an alternator, so no diodes to go bad. The circuit works perfectly when the engine is not running. Could someone suggest how to suppress what I think is interference from the car’s electrical system?