#1 If this only happens when the toothbrush is on, the pressure sensor plate doesn’t get a connection.
It’s a a little metal spring with two contacts pressing against the motor housing and the contact opens if too much pressure is applied.
Try cleaning these contacts and the opposing area if they still visibly connect. If they are bent, you might have to take apart the whole thing to straighten them.
#2 I had a similar problem where the red light came on even with the toothbrush off, completely draining the battery over night.
Opening it up, it seemed that the metal spring created a short between the upper frame and the battery.
Wrapping the spring in some duct tape seems to have fixed that issue.
After the problem came back for me after a while, I took a closer look at the toothbrush.
The reason for the LED lighting up is actually a short between the metal cage and the feed wires for the LED.
These were bent into the direction of the cage in my case.
Bending the wires up now permanently fixed the problem