My 1989 SL300 Auto is undriveable. The old style cobra radio remote key does not unlock the doors (green button), nor does it operate the immobiliser. Nor does the yellow button operate the panic alarm. The red LED on the dashboard is permanently off. When I unlock the car door with the mechanical key, the intruder alarm does not operate. When I insert the mechanical key into the ignition and turn it on, there is nothing. My whole car is dead. Under the bonnet/hood, I have turned the key on the siren to the off position, but still no joy (this would only de-activate the siren, i believe).

Is it possible that either a) the battery back-up to the siren/immobiliser is dead? or b) both radio keys are faulty (both have new batteries)? I don't really want to spend £500 on a new alarm system.

SL300 1989 Automatic, 140,000 miles (and running like a dream), Old style cobra alarm (Cat 1) 4-S-5320-STC: link to the black with green and yellow button radio key style: http://www.drivenincar.co.uk/shop/products.php?g1=ca379f