Ok let iron this out, I have seen a lot of things like this, I spent over 20 years in the auto electronics biz. Starting with the fact that you don’t have a dead short if you did you would have a box of blown fuses or a car on fire, what you have is a high current draw somewhere in the car. but starting with the easy problem the taillight is going to be a broken wire or bad socket the harness come down the driver’s side feeds the driver’s side lights then goes across the trunk to the other side nothing there to cause a draw.
Do a good battery load test, if you can’t at home take it to the local ap store, I have seen brand new batteries come out defective.
I wasn’t clear on when his started did it start right after the radio install? if so take out every inch of the new radio wiring and recheck it, but for a radio to draw a battery dead in 2 days it would almost have to be tuned on, the only way that could be done at the point of install is somehow sending power to the "power antenna lead" on the radio this should be taped off and not used.
if the problem was there before the radio then check the whole system for a draw, the old school down and dirty way is to take a 12v test light the kind that uses a "dome light" style bulb, not LEDs and disconnect the positive battery terminal and with the ignition off connect the clip end of the test light to the battery and the pointy end to the cable, some tape may be required, if the bulb comes on there is a draw the brighter the bulb the bigger the draw so it may be on very dim.
now in that car there are only a few circuits connect the constant on side of the fuse box meaning they have power when the ignition is turned off, and they are the only ones we need to look at, the radio memory, dome lights horn are a few of those, it’s not going to bet the heater for example because there is no connection when the ignition is off. Start by pulling the fuses one at time to see if this clears the draw. Then go through that circuit, but it probably won’t be that easy.
check all the components under the hood there are things connect to the battery that don’t go though the fuse box, alternator is always a good one, bad voltage regulators will charge the battery but not shut off when you turn off the engine. Some of the ignition modules disconnect these one at a time and check the draw.
you can look at that steering column check the connector it may be forced in upside down the ignition connector will only go one way but the last time I had mine apart it looked like it could be forced that way?
just remember this is going to be something that is "on" or partway on that causing this, if you have a wire " shorted to ground" it’s going to blow fuses or burn even a slight discoloration in the insulation might show that but don’t waste too much time looking for that it would be almost impossible for that to drain your battery
hope this doesn’t confuse you more that help, electrical is one of the things most mechanics don’t understand, that why I was able to make a pretty good living solving problems like this for them
Jim
