I don't know what cause the original motor mount to fail. I thought about different things that I might do that would cause excessive strain on the mounts.
On one of my first test drives, I do recall having the emergency brake on for a few yards. I'm sure that would stress the mount.
So, I looked in the Haynes manual and found the wire connected to the emergency brake switch. The switch goes to ground when the brake is on. I connected this switch to the switch on the throttle potentiometer. So, the wire from this switch is grounded when the brake is on and the throttle is pressed.
I had a 12 Volt DC Piezo Buzzer from Radio Shack. I connected one wire to this switch and the other to a fused 12 Volt source. I should hear a buzz if I should do this again.
Now, I'm trying to think of other scenarios that might cause excessive strain on the mounts.