First of all, find a new mechanic. Never go to the dealer - they overcharge and often don't even fix the problem, or worse, break other things.
i assume that the seat, locks and windows is unrelated to the wiper/turn signal issue? i.e. the seat/windows/locks never work, regardless of the turn signal lever position? This just all started happening around the same time?

Let's start with the basics...

Since you have an under-voltage error, check you vehicle battery. You should have 12.6v at rest, no less than 11v while cranking, and about 14v at idle. If you have much less than that, it may be time for a new battery, or to check the alternator.
Check the fuse for the door controller. It's #21 on the cabin panel (end of the dash, driver's side).
I suspect that the problem with the blinker/wipers is that the combination switch (i.e. that stalk) is bad. Did your mechanic reset those fault codes and then try to re-scan? If so, did they return? If he did no try to reset them, then GET A NEW MECHANIC!
What exactly is the issue with the trunk? If it is not latching, then it's probably a bad latch mechanism and will need to be replaced the part is about $100.

 

The fuse was ok. The mechanic that replaced the transmission right before this happened Said maybe master window switch was causing the windows not to work? He nor I realized that the seats didn't work till I got home. He couldn't fit me into schedule for a while so I went to another mechanic who was the one who told me to take it to dealer..... I don't have the money for that . Dumb question but do you think if the battery is bad, that would have anything to do with the seats and windows not working? Or if when I got my speaker subs put in in back if that my have caused some kind of short in system? I'm so frustrated with this car. Ty so much for your help! Beyond Frustrated and know nothing about cars.

 

How long did you leave the fuse out? I might leave it pulled for about 5 minutes. This will reset the door controller (a small computer in the door that interfaces with the lock in that door, the seat controls panel and the door control panel) Oh, one more test you can do for the door controller - does the mirror adjustment work? If not then I think this is definitely an issue with the door controller.

Rarely would the driver's windows switch panel fail in such a way that the entire thing fails to control any of the four windows or open the trunk. I don't think that mechanic is the right one to solve this problem, either. You really need a MBZ specialist to look at this - one who understands the electrical systems. But not a dealer! They charge WAY too much and will just swap parts until errors go away.

Your car is full of computer systems that all talk to each other via a CAN bus. For example, when you press the button on the driver's door to lower the passenger side window, it is not directly energizing the motor in the passenger door. Instead, the button on the driver's door panel connects to a small computer called a Door Controller, inside the driver's door. That unit interprets the button press and then sends a signal out over the CAN bus that says "lower the passenger side window". All the various systems in your car are connected to the CAN bus, but only the one that needs that command reacts to it. That would be the passenger door controller. A similar thing happens when you press the door lock button on the dash or the SmartKey fob. A signal is sent out over the CAN bus that says "lock the doors" and everything interested reacts: the driver's door controller actuates its lock, the passenger's door controller actuates its lock, the rear SAM actuates the trunk lock.

These computers are very sensitive to voltage fluctuations. Very low voltage, or even voltage "spikes" from improper jump starting can cause erratic behavior or even damage to these computer circuits. Maybe the audio installer shorted something that caused this damage. The shop that did the transmission work might have needed to jump start the car and caused it. Or maybe the door controller just dies (it happens). As for the turn signal stalk, I think that just broke as it's not an uncommon problem, tough not a regular failure.

So, do what I said - check voltage.. Also check the mirror operation, and leave the fuse pulled for 5 minutes. Heck, it wouldn't even hurt to leave the negative lead on the vehicle battery disconnected for 30 minutes or so to "reboot" all these computers. When disconnecting and reconnecting the battery, just be "sure" when you do that. You might get a brief, small spark, but it's normal and not dangerous. Don't "stutter" the connection, creating fireworks - that's bad.

 

Will do Rodney. Thank you so much for all your help!! I will check back.