Mmm hard one this, go back a few of your posts. Remember the Gmc gathering on the 05/06/14,we where there. Not sure if you were . Its not very often that 30 odd Gmc's get together but I had a good look around the one's that bothered to turn up and I'm not sure that ANY were of original spec. There was one running DUKW wheels and tyres,another had been gassed and the burble of a diesel motor. Very few were as they left the factory, ours at some stage has been 12v converted but all of this does not detract from the fact they made it to Normandy some 70 years on. I'm sure a lot of people have their little secrets under the hood or seat or just covered by a tarp. Really it matters not, the vehicles are on the road and the owners derive pleasure from that.
Anyway my best 12volt addition would be satnav, mobile dvd player oh and yes my mobile phone charger.....