Jaguar xj x358 fast wiper speed problem

[Steve1066]

Hi All
I'm new to the forum and could do with a little help with a fault on my x358 if anyone can.
My problem is that when I select fast wipe on windshield wipers the speed does not change. I have swapped over relay 8 with relay 4 in the engine compartment which makes no difference at all. I've took the can off relay 8 and operated it manually by pressing the relay down while the switch is in the fast wipe position and this works, making the wipers go faster. this tells me there is nothing wrong with the motor. Also the fast and slow demand signal come from the same wire off the wiper stalk switch so I'm assuming that the stalk switch is ok as the slow wipe works fine. It seems the fast / slow relay is not receiving the demand signal when I select fast! 
has anyone any ideas what my issue might be? any help would be appreciated.

Steve