Earthing on Land Rovers, especially those that are mostly alloy is always a possible cause of intermittent faults but mostly cause a failure to operate rather than the symptoms you suggest. Sounds like the speed of the wiper and blower motors is changing with the balance between load and supply. Quite simply if you rev the engine and your alternator then speeds up you will generate a better supply therefore you may find that the motors speed up with engine speed. Similarly, if your vehicle has an electric radiator fan or heated rear window then the motors slow down when these are on.
If this is what is hapening then this would point to a weak battery or a faulty voltage regulator, basically
your vehicle is reponding to how much juice the alternator is putting out rather than a nice steady 12V+ from a well regulated healthy battery? If you know someone with an electrical meter then ask them to measure the volts at some convenient point and the nearest bare metal. Rev the engine to see if it changes much. Suitable point would be an interior light fitting. Take the lamp cover off, remove the bulb and the end with the wire attached should be positive, normally the other side of the fitting gets earthed via the fixing screws to the body.