electrical gremlin

[67gmcss]

I have a complicated (to me anyway) electrical problem. In my 67 gmc step side, with just the parking lights on, if you turn on the left turn signal the arrow on the dash lights up, but dimly and won't blink. Up front, the turn signal goes completely out but the rear works fine. If you turn on the right turn signal the dash and all works perfectly except that BOTH rear lights blink. What in the world could my problem be? Thanks in advance for any help.