I bought an MSD 6AL recently which was dead, These things seem to be uber sensitive to voltage spikes, bad connections, disconnecting them when the battery is still on the vehicle, which kills them.
After researching on the net, and taking the box apart, i found there is an internal fuse which dies at any sign of trouble.... most people think their box is dead and chuck it, or send it back to MSD to get charged for the repair (they tell you its not possible for somone other than their techinicans.... - which is ********, they just want your money (between $40-$80 for an easy repair)
It looks just like a diode, so doesnt jump out at you as a fuse. see pic..
Its the black one on the right of the thickest red wire.
You can replace it with a new one, or you can bypass it with a piece of wire, but i'll be modding this one to use a blade fuse, which can easily be seen/replaced.