when all looks good at the ends where it counts, yeah next step is to start digging deeper. im no electrical pro by any means, but i do know in the right hands multi meter can lead you to your failure point most of the time.
could start looking at the harness for areas that rub on body parts, could have worn through in a spot and broke a wire or two.
do you have a simple test light? if so pull the side that works, unplug the turn signal and see which wire is live power when the the blinker is on, the test light should flash,(key to on) then try the same pin on the side that dont work, to rule out the socket all together, i do know that sometimes when one wire goes bad it can affect another thing in line with it, aka the drl.
not sure if you have or not, but as odd as it may seem, check the rear bulb as well you never know, the tails seem to have a moisture problem on some cars.
