I'm doing a spot of re-wiring and have always wondered - is there any science behind whether a load should be switched on the negative or positive side? Over the years I've come across both methods on various makes and models...sometimes a mix on the one car. Thinking of things like wipers, horn etc. Anyone know for sure?

Makes no difference, but convention has it on the +ve

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Too many cars, and too little time.

It saves on copper, you don't need to supply a +ve feed top the switch and then a return wire to the horn/wiper etc.

In industry, wipe 10 cents off the each of a 5 million item stockline, and you will be feted around in a sedan chair with rose petals thrown out in front of you by naked virgins.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Rose petals eh? mmm....

That's kinda what I was thinking. I have the horn sitting right next to a permenant positive, so makes sense to earth through the switch, whereas wipers it's easier to run a switched live across to them and earth there. Prolly a few other examples as well....

Thanks

With high load devices (like wipers, lights etcetera) it is important to keep the length of copper the current travels on to a minimum. Calculations are made when designing the cars as to current carrying capacity, reliability and life expectancy of each component. They come down to simple numbers like: failures per million hours or failures per million operations.
So minimising copper length means minimising copper load (as you would need to increase the size of the wire to reduce copper losses) and reducing wiring complexity means lowering costs.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.