grid heater and oil pressure

Are you saying that your wait to start light isn't coming on at all or it just isn't longer than the normal bulb check when you turn the key on? Your grid heaters should have come on by now and the wait to start light should be staying on for quite a while by now. I forgot at what temp they actually come on but I know mine do when it is in the 40's.

You can jump the relays for the grid heaters but they probably work just fine, not sure about doing anything else. I think the wait to start light is an LED so it probably isn't bad. I'm afraid to say that I think your ECM might be going bad.

you might get lucky and it stay working for a while. It controls everything to do with the engine, if it dies your truck shut off. You'll need to make sure that the part number on it matches the part number on your ecm because there are some differences in the years.

If the wait to start light is not coming on right now the damage is already done.
I doubt that the sound system had anything to do with the demise of his ECM but maybe.

I would be checking to see whether the alternator is putting out any AC ripple with both the sound system off and cranked up to see if there is any. The ECM seems to be extremely sensitive to noise.

300 amps is a HUGE load and you will see a drop even with good batteries and good alternator, it might not be to 8v but mine will drop to 11-11.5 while cycling. From outside it almost sounds like the AC compressor is turning on and off

300 amps is a lot but a 290 amp alternator isn't get near the drop that the stock alternator would obviously.

You could use a clamp ammeter but most will only measure AC current not DC or if you don't have one just check the voltage across the grid heater to see if the relays are coming on, more than likely the element itself is fine, if you get voltage and no heat I would either get a clamp meter and measure the current or measure the resistance if the element. It should be pretty low. If you don't get any voltage at the element it self check the voltage on the coil side of the relay coming from the ECM