blimey,what watts does it draw on/in standby.
in my caravan i leave tv with dvd and hd hard drive satellite recorder on/standby all the time,never run out of power.they draw 5.5amp when on,2amp in standby.
is your battery on way out.
My boat 16" Tv /Dvd player takes less than 1.3 amps when on....
We have to be careful of energy as we only have a small outboard that charges at 5 amps at normal cruising speed.
We have two 100 Ah leisure batteries and I have installed battery and current monitoring to each battery, and never discharge them lower than 80% of their total capacity.
You can buy "piggy back fuse" holders from ebay, that replace one fuse, and gives you an extra fuse output, so choose a fuse that is only powered when ignition is on, eg, cigar lighter, and go from there.
You should be able to "fish" a cable from behind the roof lining, without removing it, to the front drivers "A" post and down to the fuse box from there.
I doubt that the lighting cable will be capable of taking that extra current.
Also consider the earth return, local body earth to a suitable earth point, or run it back to the fuse area.
You will need cable that can normally carry 15- 20 amps with that load, to avoid any voltage drops.
If you want to consider watching the DVD when parked up, then, consider a second battery that is only charged when the engine is running, otherwise you might be stuck. Then also consider which fuse feed to take, maybe the radio, or one that is energised when the ignition is set to accessory.
Note... you might find that the Dvd doesn't like voltage drops when the engine is being started... some accesories switch off when in " starting" position.
I do think a second battery, is the way to go... eventually.:thumb2
Rustic