I wonder if the polarity of the socket is connected up incorrectly. There are two connectors underneath the socket, if they have been mixed up then it will explain why it's damaged your TomTom.
On some cars the socket has been known to come loose and pull straight out
. If this has happened to the previous owner (assuming you didn't buy it new) maybe the connectors came off and were reconnected the wrong way around.
But its a T Shape connector, so impossible to plug in the wrong way around... Possible the cables have been crimped in the connector during manufacture.
Interesting the fuse has not popped...
My guess is that either the Tom Tom has developed a fault itself and drawing to much power... The accessory circuit is protected up to 10A and The Tom Tom charger converts to 5v 2A (USB Standard)... If the charger has gone wrong, then this will fry the Tom Tom.
Another possibility is that something else is drawing power form the Accessory circuit. Maybe the previous owner had something fitted (Tracker, parrot etc.) that's hidden away under the dash that you cant see.
I know this should not cause the other items on the circuit to overheat or draw to much current, but something weird does go on!
When installed my first TV unit to the car, it had an internal fault (unknown to me at the time), so every time I connected to the accessory circuit, the wires would smoke and get very hot, BUT so did whatever else was plugged into the other ciggy socket (phone charger )..... but the loom going to the socket was unaffected ???