SaG;
As I found after dissecting a virtually brand new switch that failed, there was no snap-action or wiping action with that simple design of the contacts...that, combined with how slow they close because of the slowly rising pressure is THE major factor why these switches don't last...in general, switch contacts are designed with spring mechanisms to provide fast-make/fast-break in order to 1. cut through surface oxidation to assure current flow, and 2. prevent prolonged arcing and the resulting heat...unfortunately not here, because the diaphragm simply follows the system pressure...not to mention the inrush current of incandescent lamps can easily be double the steady-state current...so when you say 2 X21W (3.5A, scale that up even more when adding trailer lights!), that's only what current settles down to after an inrush shot way more then that...all these factors combine to force the switch to operate in a worse than worse-case scenario, which leads to an early failure...
I do have to disagree with your suggestion that corrosion somehow increases the current...It Cannot! Any poor connection in series with a load in a simple circuit as this** acts to increase circuit R, this drops Voltage available to Lamp and thereby decreases I (which decreases brightness, reference: Ohms Law I=V/R).
** In complex circuits, where the amount of energy pulled from the power source is
constant due to some controlling circuit assuring that voltage being supplied to load and therefore current through load are
constant, total circuit current would be increased by a series resistance (ie corrosion), but this does not apply in the case of a simple circuit, consisting of a power source, switch, and lamp.
Mike;
"Surely after 50 years they could figure out how to make a switch last longer than 6 months!" I doubt any manufacturer is interested in spending one dime in doing any continuing engineering or improvement on these switches at this point. There is just no return on that investment! ...so are happy selling you replacements of that terrible (oops! I almost said POS!) switch 'till the cows come home...
"I'm torn between the desire to keep the car fully stock" if you mean stock for year of manufacture of an earlier chassis, adding the bracket wouldn't be...BUT, stock, as what the factory produced, it absolutely would be! ...because the bracket with PPSS will absolutely look the same as the later chassis which also had it! A concourse judge making an inspection of the engine compartment would not see much difference!
VOC member Amazonauto had similar concerns of originality (reference thread:
http://www.volvoforums.org.uk/showthread.php?t=118895) ...I invite him to chime in here with his impressions...but here is the finished appearance of his efforts...absolutely stock!...to go along with his spotless stock engine compartment! Pic used with permission:
Cheers