Does everything have to be so complicated!

Number Plates.
This applies to your own privately owned car only.
Go to Causeway Post Office, counter No. 15 or 16. ask for a form to change
Number Plates. This is the same form as used to cross the border to SA. Its free. Ask if they have the correct shape plates for your vehicle.

Then fill in the form as if you are going to SA. And take it to Southerton
for clearance, When they take the details for entry into their book tell
them its for number plates only. Take the log book with you and your I.D.

After this is completed take the number plates off your car and go back to
Causeway Post Office, to the same counter. Take the log book, a copy of your I.D. or drivers licence and the original the number plates and $360-00 to pay.

Ask at your first visit to the counter at Causeway, what you need to bring
with the number plates, in case I've forgotten or anything has changed.

The first visit takes about 2 minutes unless you have a big queue, and the
second about 5 or 6 minutes again depending on the queue.

I found it best to go to Souderton after lunch, they start work at 2-00 pm
"Central African Police Time". It took me about an hour in total, with 7
cars in front of me in the queue. They keep everyone outside until they are
ready to start. I suggest you don't go just before a school holiday