Just keep at it... use headphones, get a stereo to mono adapter for them and cut out the left or right channel...if you have an E.Q., try to isolate the frequency range that the guitar is playing in.
I don't know if it's still possible, but I used to go searching for 12" LP remixes and instrumentals too. Some of the remixes are great cause the engineer will isolate tracks that are way back in the mix. Maybe check the video store for live versions if you're really stuck.