Could anyone recomend good exercises for my external rotators. I had a slight burning sensation around my left shoulder blade, and a weak external rotator was the doctors diagnosis. I just need to strengthen it up.
There is no "external rotator". The muscles responsible for external rotation are the rear delt, infraspinatus and teres minor. You can do plain vanilla external rotations, bent over flyes or rows to the neck. These are all generally done with dumbbells or cables. If you're smart, you'll make them a staple in your routine. And for the record, no back is complete without a well developed set of rear delts.
It's also very possible you've overdeveloped your internal rotators: pecs, anterior delts, lats et al--and may need to do some stretching/foam rolling for them to balance things out.
Heavy lifting on the basics (pressing in particular) combined with frequent internal-rotation in everyday life (driving, slouching, typing on a computer) often results in chronic internal rotation...sort of an occupational hazard. But the exercises above, combined with some postural awareness and stretching, should help: shoulders will stay healthy, you'll stand taller and lift heavier.
Plus your chest will look bigger. Not a bad tradeoff.
Investing in the Buchberger-12 manual or Inside Out DVD (or both) would be well worth it.