I thought you had registered?
And where's your signature telling us about your car?

Jumping the right side taillight to work the left side one SHOULDN'T drain your battery but... these 240s have some pretty clever circuitry built in so the voltage doesn't always go where you think it goes.

As far as checking power to the left side tail lights, check this out:

Wire arrives at left light assembly via white nylon connector. Connector clips to circuit board. Connector opens up; it has some kind of clip that lets it unfold sorta like a cigarette box lid if I recall. Anyway, you can open it up and find the contacts that meet the circuit "board". They have 2 fingers. Look like silver, maybe they are. Clean points gently with emery cloth. You can pinch the fingers together just a bit to make them grip the board better. I'd probe the contacts to test for power there. Or maybe strip the wire just a bit to test in the wire itself.

Of course you wanna know which wire to test. That's where you need a book...

If you can't get your hands on a Beneley's book for 240s, at least get a Haynes. They have pretty decent circuit diagrams; you can see how most of the lights are wired and starter/charger and also power door locks and power windows. Dop't bother with Chilton.