HI,
Had the same problem last year, took the car to lexus chester, they replaced the relay. The same night it was the same, lights coming on and going off intermidently. so took the car back. 2 days later they phoned to say the had fixed it again, then asked for £250ish for a new boot seal they had fitted, i told them to run and jump for the cash as the car was still under warranty and they never asked if i would be ok to pay for the seal.
But the seal did work. they said water had leaked into the wireing at the inner quarter and started to rust causing the lights to fuse.
hope this helps.
