Electrical issue close to getting solved?
 

Connected a light pen from to neg cable to neg post to observe when light would go out when fuses were taken out.

Discovered the removal of 9 would make the light go out, but ..

only when C and 9 we both out at the same time.

If 9 was taken out, but C was left in, the light would stay on.

9 controls the diagnostic module, among many other functions.
(Started with http://www.peachparts.com/shopforum/tech-help/366497-check-engine-light-after-key-out.html)

Noticed the transmission selector lever position light is not working - controlled by 9 also!

What does this mean and what should I do next?

Might help if you said what the electrical issue was???

Or in other words, what is the problem with your electrical system?

Started with the check engine light staying on - even when the key was out.

To the point where the battery drained overnight - regardless if CEL was on or off.

First, I was focused on resolving the CEL - but I have no fault codes now.

Am trying to pinpoint the battery drain, but am confused about the C and 9 combination.

Ok, you are best to use an amperage meter. Some things in cars are always on and always use a little electricity. Like a radio - needs to have "standby" power for the memory to remember the stations selected.

So those things will use very little electricity from the battery. A malfunction which will drain the battery will be using a LOT of electricity. You can tell the difference with an amperage meter.

Thanks for explaining the details on why an amperage meter should be used in further analyzing this drain.

In researching, found a thread on the radio causing the drain:
http://www.peachparts.com/shopforum/tech-help/368174-battery-drain.html

Is this situation possible for an aftermarket installed years ago with no problems but
now causing a drain?

In other words, if the radio was installed a long time ago and the battery drain is a new problem, should the radio still be examined as a possible drain source?

Anything can cause a large battery drain if there is a malfunction. That is the reason to use an amp meter so you can trace the problem to its source - whatever it is.