Re: Why my bridge single coil sounds harsh and lots of treble?
Linking the bridge tone to another pickup is highly undesirable for me. There are no two pickups on a Strat that I ever want being at the same tone setting, so doubling up pickups on the same tone control necessitates constant tone knob fiddling with the way I play. I absolute hate G&L's PTB system for this reason. Makes everything a master control...and I never want a bass cut on a Strat anyhow.
Even if I did want or need a tone control on the bridge pickup, I would rather pull it from one of the others in order to get it...or wire the Strat for T/T/T, and give up the volume knob (which I almost never use on a Strat). I'd pull it off the middle pickup if I thought I needed it on the bridge. I rarely tamp down my middle pickup, so I could live without a tone knob there.
I set my Strats/amps up so that the middle pickup is the "home base" tone. Then the neck is one darker, and the bridge is one brighter, and you're good. There's no situation in which I would want the bridge pickup, but with the tone rolled back; I'd use the middle pickup if I wanted that. I only use the bridge pickup when I want a ton of brightness...e.g. when deliberately using extreme "twang," or driving the amp into breakup – to help retain clarity. Same reason (but less extreme) that you'd use the strangle switch on a Jaguar: to remove low end for a cleaner output from the guitar, which then retains more clarity when it heavily breaks up in the amp.
The other key is lowering your other pickups so everything is more balanced in terms of output.
FWIW, I wire my Strats without notch positions (3-way switch). I never use those tones (I don't like the frequency notching), and they just get in the way of me quickly returning the switch to the middle poisition while playing.
The Strat and the Jazzmaster are the two things that Fender got totally right within the first few months of production, IMO. For the things I play, vintage Strat and Jazzmaster wiring cannot be improved upon.