Remote control for retrofitting ZV... But how?

Hello.

I have an Audi A8 built in 1995, and would like to retrofit such a flipkey remote!

I've already found one on Ebay, looks like the original equipment!

But how do I best build this one, where do you clamp it?

I know this with the Audi 80, it was the easiest under the back seat, there were all the necessary cables, where can I find them with the A8???

Thank you, greetings.