There are NO stupid questions.
The ECM is not the fusebox. It is the Electronic Control Module for the engine...basically the computer. The problem, as I understand it, is there is power at the injector at all times. That power comes from the ignition switch, NOT the ECM. The ECM controls the ground side of the circuit.
My only suggestion at this time is to check the voltage on the white wire on that injector connector with the key off, and the injector unplugged. If there is ANY voltage there, there is something that is giving it that voltage...there shouldn't be any voltage there with the key off, period. If you replaced the engine compartment harness, the problem will most likely be either the ignition switch itself, or the wiring under the dash.
Is this something that just started, or has it been a problem for a while?
Has there been any work done under the dash?
Is there any voltage present at the other injector with the key off?

Here's how the injector circuit works:
Key ON, the ignition switch sends 12v(or battery voltage) to the injector on one terminal. In order for the injector to fire, it must have a ground. This ground comes from the ECM(computer). The ground is what is known as a PWM(Pulse Width Modulated), meaning it pulses very quickly, and varies the pulses depending on engine speed, and other information gathered by the ECM needed to pulse the injector at the right frequency to keep the engine running at optimum performance. That's why they "click" when the engine is running. The ECM is turning it On/Off repeatedly, extremely fast.
So....in order for the injector to be the cause of a battery drain with the key off, there has to be something else on that circuit feeding voltage to the injector. It could be the ignition switch itself....or it could be some other wire that is "hot" with the key off that has shorted itself to the wire on the injector circuit.
I hope some of this makes sense.
I can only say that you really need to start at the injector, and work your way back to the ignition switch, looking for the problem spot. If you have already replaced the engine compartment harness, we can eliminate that as a suspect. From there, it will go thru the harness connector at the firewall, which is really the fusebox on the inside of the car. I have seen the fusebox connector melt enough to cause shorts/cross-feeds in the wiring harness, but it is rare.

Joel,

You are THE man!  Thanks!

This problem has been around for a little while now.  I had a similar problem with a battery drain some time back and tracked it down to the power seats.  I just removed the fuse (since nobody drives my 'Vette but me and so the seat stays where it is) and all was fine for awhile.  Then this current stronger drain showed up.  As far as work under the dash, there's been plenty.  I've replaced several items under the dash over time. 

Don't know that I said this or not, but the drain does NOT occur if the harness is detached.  Does that mean anything? 

Thanks for the help!!!
Jeff

Dis-connecting the injector will stop the drain because you've eliminated the voltage going to the injector. The other wire in the connector, the ground signal coming from the ECM could be shorted directly to the frame, and it wouldn't drain the battery....because there's not supposed to be any voltage there(from the ign switch) to start with, with the key off.
I'm assuming that the car starts and runs fine, right? That would pretty much eliminate the injector as the problem, as far as the drain. The injector could be slightly grounded internally, but with the ign switch voltage being "killed" when you turn the key off, it would only effect the way it runs, but it wouldn't cause a drain.
You've got a wiring problem under the dash, or the ignition switch is bad....that's really the only two things it could possibly be....

I agree Joel.  My bet it's the ignition switch.  Sounds like another project for me this weekend!

One thing I've been forgetting to mention...IF the injector has been powered up all this time, and draining the battery, it's possible that it may fail soon, due to heat buildup. With the engine running, the fuel flowing thru it will keep it cool, but without the fuel, the injector will get hot...especially since it's just full "On" all the time, instead of being pulsed by the ECM.
Just something to think about...

It's always something, ain't it?
Adams' Apple2010-01-28 20:45:21

Yep...it's definitely always something! 

New question: When I go to switch out the ignition switch, will I have to remove the steering wheel to get inside the column?  My 'Haynes' book says I don't...but I'll be danged if I can see or figure out how to get the column open without taking off the steering wheel first.  Am I going to have my hands full replacing that ignition switch?

Thanks Joel for all the help...you don't know how much I appreciate your great advice!

Jeff

The ignition switch is down on the lower end of the column...no need to remove the wheel. You will need to be a contortionist, however!
There is another thread on replacing the ignition switch here somewhere, with pics of the switch. Do a "search" and it should come up. I can look for it too, once I get to the shop.

Awesome!  Will do!
Thanks!

Thread 1
Thread 2
Search results

Pic 1
Pic 2

Can't thank you enough, Joel!  I'll keep ya' posted!

Jeff