The first task is getting to the battery. As I’m sure you know the battery is in the trunk of the Retro. So my suggestion is to use the “secret emergency key release” to open the trunk. This is located in the wall behind the driver’s seat. If you read my Story #1 above you’ll know why I suggest you open (test) the trunk release this way.
Once the trunk is open I remove the fiberglass floor from the trunk and then remove the spare tire. This is also a great time to check the condition and air pressure in the spare tire. The only tools I needed for this job was a ratchet, long extension, a 13mm deep socket and an 8mm deep socket.
The next thing I did was to remove positive (red) terminal from the battery and secure it out of the way. I then removed the black steel bracket the goes over the center of the battery. It is bolted to the side of the trunk well and passes over the battery and down through a plastic lock block that holds the side of the battery snug. Next I removed the two bolts holding the metal brace on the floor of the trunk at the front side of the battery. Sliding the battery towards the rear of the car made this easier.
Once that is removed you can slide the battery forward to give you access to remove the negative terminal (black) and the rubber vent hose that slips onto a port on the negative side of the battery. The battery should then slide out with a little muscle.
This is a great time to check the floor of the trunk for any dirt or signs of battery leakage. Clean as needed.
I then just did the above in reverse. A good sign that all was well is that the trunk light was on when the job was completed. After tools were cleaned and put away I did the last task of resetting the radio and clock.