Here is all you need to know to build a PWM circuit. Thanks to XERO for finding this site.
Here is the rest of what he mentioned about it:
"basically, if you can't figure it out from that, you prolly shouldn't be attempting it.
That is the model I'm using for mine, works like a charm, just hook the output pin up to the signal pin on a transistor to activate the LEDs.
That will get you flashing, atleast. (There's a link for a nifty dowloadable timer calculator so you can figure out values of resistors and capacitors needed)
The way mine is set up is, I have diodes on the 2 positive(+) inputs, and I flash the LEDs at half their rated voltage. When flashign at half voltage, it's as dim as it should be, when the brake light constant comes on (also at half voltage) it brightens up very noticeably, and a constant half voltage is bright enough in the day time for brake lights.
There wasn't a significant enough change to just "stop" the flashing, also I couldn't figure out how to do it, that's why I'm doing it this way.
anyway, just noticed alot more PWM questions, so thought I'd share (and to think, before I got here, I didn't know jack, lol)"