You might be lucky and be able to get to the ECU by just lifting the carpet from the front. [Saves all that seat stuff]
Lift the mat and then the rubber trim from the door plate. You can pull the carpet up at the front and hopefully pull it back to clear the plastic floor box. [Thanks to Marcin for that tip]!
Be aware that the BT ECU is not a straight replacement for the Motorola ECU ...
Julian