Arthur,
From the service manaul for a ml23934 system (guessing this is what you have);
"If the park brake is set, check the 9000 (BLUE) wire in the 4 pin MTA plug at the panel for a ground. If there is no ground, the 9000 (BLUE) wire or the park brake switch is the problem. If there is a ground on the 9000 (BLUE) wire, make sure that wire and the small jumper wire is pushed into the MTA connector properly. Check voltage at the panel between the 6120 (RED) wire and the 9000 (BLUE) wire while pushing the "ON" button. If the voltage is less than 9 volts, the panel will not turn on. Check all connections on both wires, a poor ground on the 9000 (BLUE) wire can cause low voltage for the panel. At least +9 volts is needed to turn the panel on.
If the voltage is greater than 9 volts, replace the panel."
Steve