The front tv is not supposed to work while driving. I would not disconnect the safety feature as it could lead to a fine or worse cause an accident. Not famaliar with the Seneca but I would take it back to the dealer if the rear tv will not work if it is supposed to. Most rear units will while the rv is running.
Starting the engine will open a relay in the 120V power supply to the front TV in many motorhomes because the law in most states requires that any TV that is viewable from the driver's seat must be disabled. The rear one should operate just fine as long as there is 120V power, which won't be available unless you are plugged into shore power or your genset is running. On the front TV there is a fuse that can be removed if it is properly wired that will prevent that relay from operating. It is pretty easy to do but you will have to locate the fuse.