make sure wires are right way round, yellow to white sender, blue to black/brown/blue sender. But if the light comes on above idle its beign caused by the high pressure circuit, which should also set the buzzer off. Since its not, I'd say the oil pressure warning light circuit is kaput, its the 'L' shaped bit clipped to the back o the speedo. Tbh, try another set o clocks from any MK2 with the same number of plugs, if light don't come on then you know its the clocks.
I say any MK2 as you can get carby ones for dirt cheap to try, but 16v ones usually come at a premium price!
