First off, if you use the "test" light method, you won't "weld" anything as the light in series with the negative cable will act as a current limiter. A dead short will cause the light to glow bright, where as a small drain will cause a dim light.
As far as your resistance readings go, you must take a look at the "big picture". If I take a normally operating vehicle and disconnect the battery, then take an ohm meter and measure from the positive cable to chassis ground, there are many instances you will see what appears to be a "dead short" to ground. Turning on the headlights for example will cause this. The minute you pull the switch, you link the positive feed thru many filaments in the bulbs to ground, this looks like a short to your meter. Same thing if you measure the "S" terminal on the starter solenoid to ground, your meter would read "through the windings" and see a short to ground.
At this point, if you have a 20 amp draw, that is a serious draw. I would focus on the the old divide-and-conquer method to split up your electrical system. The 2 big suspects right away would be bad alternator and bad starter.
I'm not sure how the update series works underhood, (I belive they mimic the 1972 and later chevelles where all the power for the car is "tapped" off of the battery (+) lug on the starter solenoid), if that's the case, then what you need to do is:
Leave only the big battery (+) cable connected to the starter solenoid's battery stud, remove the feed(s) that supply power to the rest of the car. Check for a current draw. If there is a current draw, the starter is bad.
If there is no current draw, you add one connection at a time. If there is only one other connection to the battery (+) stud, add it, and check your current draw. The next step would be disconnect the main (+) lug on the back of the alternator to see if the draw goes away.
remember to completely disconnect the battery before each installation and removal of wiring components, then reconnect when your test equipment is set-up properly
A 20 amp draw should be quick to find, that amount of current and you, by right, should be smelling/feeling something "hot"!