Decoding box Renault 19

Hello you outside, have the following question and hope one can help me further: Drive a R19 convertible, Bj 94. Have a central locking system with remote control. Also a factory immobilizer. Unfortunately I no longer have a plip. A friend of mine has a decoding box with the corresponding plips. For “small money” I could acquire this and use my central locking system again without a key. However, my friend does not know how to exchange the decoding box! Where is this box actually located and how expensive is the exchange? Think if I go to Renault I will pass on the resulting costs. Would be great if someone could give me information. Thank you very much to you!!!