As said above sounds an earth issue, check and make sure the earths are clean, more than likely they are corroded and you then end up with the situation explained above, basically electricity wants to get from positive to negative as quickly as possible and will take the shortest path possible, so the corrosion will add resistance and so it takes another path in this case illuminating your lights, a bad earth will also cause the fans to act as they are.

Also the fan loom was substandard to start with and this is why nearly all of us have rewired, the fuel pump suffers a problem with the old wiring building resistance and so the pump cannot get the power it needs under load, this can lead to an engine meltdown, you can buy the looms pre made or it's not that hard to DIY.

Martin