I still think it's all a dream/nightmare. Here's why:
Dexter finds the receipt for Trinity's car and figures that's where he'll find him. We see Trinity picking up his car, driving around and then driving away.
Next we Trinity driving down some deserted road and he has car trouble (engine!?). Trinity stops, pops the hood to take a look and BAM Dexter is there to attack Trinity.
Ok.. where the hell did Dexter come from!? If he was in the trunk of the car exactly how did he get in when the car was parked outside of a shop with numerous people around?
Not to mention if Dexter was in the trunk that would mean he would've had to wait for Trinity to stop somewhere remote so he could pop out undetected, subdue & drug Trinity, find a spot to set up his kill room + store an unconscious Trinity, use Trinity's car to drive however many miles BACK to his car (which is risky in and of itself because it's a Stolen Car) to get all his tools and materials THEN drive all the way back, set up an elaborate room (That kill room was by far the most elaborate one Dexter has ever set up. A working train set AND a working record player with the same record Trinity played when he killed all the kids!?) kill Trinity, clean up, drive his car and everything else to the dock, get in his boat and discard the body THEN dump Trinity's car somewhere. All in a matter of hours. Kind of a stretch.
If Dexter was following him in his own car that would also be a stretch because there was no other cars around so it would be obvious to Trinity if one car were continually following him. Also, when you see Trinity drive off (last daytime shot) there's no sign of Dexter trailing behind in his car.
Just doesn't add up to me. I think the last daytime shot of Trinity leaving in his car is the final "real" moment of the episode. I think everything with Dexter killing Trinity and then finding out Trinity killed Rita is a hallucination/dream/nightmare Dexter has on the plane en route to meeting Rita. Fueled by Dexter not being able to finish off Trinity and knowing he's still out there (possibly putting his family in danger). Just my theory.
Like I said though.. if anyone has read something definitive let me know and please link it.
