2005 TJ total power loss
 

Got an issue I was hoping to get some help with..have an 05 TJ 2.4L has 240k km on it and the other day while driving on the highway I noticed my voltage dropping all the way to 8 volts. At that point I was off the highway and at an intersection when the jeep shut off and would not start again. Took it to a shop and so far they have tried my battery (which is an upgraded yellow top due to a sound system) with two different alternators ( same make and model) as well as with another battery and the alternator still won't give more than 12v (even with the sound system fully disconnected). Called around and a guy told me it could be the fuse for the regulator but the shop told me there is no fuse and it is part of/controlled by the ECU. That's all the information I have and I was wondering if anyone else on here has experienced anything like this or if anyone has any ideas as to what the issue could be?
thanks!