paddy2bsure

That statement is not correct. When you start your car, the brake bulb light comes on on the dash - press the brakes, and it flashes up with "OK". When you press the brakes, the brake light switch is activated, sending power to the lights AND a message to the computer that the switch is on.. if the measured resistance is anything but 42w (21w bulb x 2) the warning light appears, as it assumes a bulb has blown. So, if you connect lights without a relay (3rd brake light or a trailer load) then the warning light will stay lit.
Mine had a problem whereby it beeped & lit intermittently when driving along, which was caused by a faulty brake light switch at the pedal - the computer thought my foot was on the brake pedal, measured the load, and it was zero, as my foot wasn't actually on the brake. Simple fix! PS always use relays for stuff like lights - the computer AND inboard relays (eg for indicators) don't like extra loads.

Paddy
A6 2.6 SE Saloon, 1995, 145,000 miles