M8, I can tell you for a fact it is a failure on TomToms part. I know this because I had the version one TT, also two of my friends had them. All 3 of ours cracked.
Where the screws hold the back together is where the problem is. All ours started the same way as yours, cracking the plastic causing the pins to not connect, running out of battery power etc.
Yours is not wired to turn on with your ignition, so you also experienced the problem of it not turning on by itself. Maybe find a switched wire instead of direct to battery.
Now, all 3 of us got new mounts for the V1 under warrenty(2 years warrenty down here). I got more than that a bit later, because I found that I had other issues and that was that bike vibration on long rides(I have done a few over 5000km) the actual pins wore holes in the contact pads. Check out this photo of my first unit!
I sent the TTV1 back and it was replaced. Then a few months down the track went on another ride and the same damm thing happened, wore holes in the pads. I was pissed off at this stage and gave TT an ultimatim, they could do 3 things:
1. Buy me the Touratech mount to lock the system in place, which would stop the vibration
2. Replace the TT V1 again, and again and again, every time while it was under warrenty until it broke down outside warrenty, then I would bin it
3. Upgrade me to the TT V2 as I had heard it had a different mount system
Within 2 weeks I had a brand new TT V2. I have not had any mechanical issues with it and it has performed within spec and I have done some more long rides. The mounting system is 400% superior.
Consequently, I have 2, yes 2, V1 mounts sitting here on my shelf doing nothing. They are available if you are interested. Unless you want to go the warrenty claim route, maybe try that first?
The Touratech mount is expensive, but it will stop future cracking, but not the vibrations causing holes, only gaffa tape or cable ties will fix that.