Please do not overload existing circuits in the hope of success and convenience.
The cabling is sized to suit the application, as I am sure you are aware.
An electrical fire (anywhere) is not nice...
I see your point, that's why the extra gadgets (USB and OBD totalling less than 5 amps) are hooked to the wing mirrors heating, which is a hi-load circuit with good tolerance. The addition didn't even blow the standard fuse so it's within operating range.
For this one I think I'll go with a switched live taken from the stereo, in my case it's a 4x50w system, while the TF should support up to 4x80w with no amp, so I believe there's plenty of margin for an electric actuator not too different from a relay ;-)