You have a start assist relay that is most likely the issue
Follow the Yellow/Red wire from the starter to the relay
Your key energizes the relay, and the relay energizes the starter solenoid
Check for 12V on pin 30 and 86 of MPR
Ground pin 85 of MPR to see if things power up
Check for 12V when key is turned to ON, on pin D of DLC connector (purple)
I have EZports which are also drive on. I go a piece of heavy foam and wrapped with carpet. Makes a bumper if we come up to fast
Can also make or buy an extension pieces (EZport sells extensions which are also dock boxes)
I would contact Bob at OBD Diagnostics and see what he can advise. Your 865926 is not listed, but if yours is bad he may be able to fix or advise other
https://obd2allinone.com/mercecms.asp
Those are casing numbers, Merc serial number would be something like 0M123456. If there it would be a plate just above the starter.
Post a pic of the ECM so we know what vintage it is
There is something miswired or something is wrong the the Petronix.
Don't like the Pet but when it does work there is no issues
It melted the wire but didn't blow the 20 amp in line fuse under the helm, doesn't make sence
Note: the B3X, is a Bravo X upper and standard B3 lower. The X part is only the upper section.
If your going to try and use it next season, clean it up and put some JB weld on it to help keep water out. Best to just replace it now