My laptop runs a sat-nav program in the car via a Maplins 15,16,18,19,20,22,24v adaptor (£19.99) plugged into the cigarette lighter. It was quite happy (no crashes of program or car on a recent 505 mile run across Spain) It didn't seem to to get hot at all on the 19volt setting.
If you have a 4 pin connector I think you will find 2 positive and 2 negative one circuit powers the computer and the other recharges the battery
I don't know if this helps but NACO the National Association of caravan owners 01255 820321 sell a stick on indicator for £2 which they say is reusable. Worth a try maybe?
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