My 72' does not have a switch for the E-Brake, it only has the wire on the proportioning valve. Supposedly the only time it will come on is if there is a problem with the brake system, meaning a change in the hydraulic pressure from what it is supposed to be. So if your rear brakes are really worn down then it could come on, also if the front brakes were worn it should also come on or if there is air in the system or a leak. Mine came on when the rear wheel cylinder started leaking, and again when the master cylinder whent bad, I replaced the master cylinder and bled the rear brakes and it went off for a while then it came on again so that probably means that I need to bleed the front calipers to get it to go off again.
