I would also say that you have an always on, a blinker, and a ground lead.

on each corner bulb there is the same three wires, you should be able to just tap into those.

just make sure you wire each side to each corner bulb...you cant wire both of them to one side. (i.e. if you wire them both to the left blinker bulb then both are going to blink on left turn signal.)

I'd like to have sidemarkers, but I'd never trust myself to put holes in my fenders.

me too.