sounds like you have done everything possible, relays shouldn't fail in that manner, i recon you should take the gamble and fit the new one, if it works take the last one back and ask for a refund. are you sure the motor is running free? i had an intermitent fault on my rear wiper motor, cost 68 quid plus vat for a new motor. the old one was siezed, i took great pleasure in destroying it when i saw the price of the new one!