Where is or how do I install the Aux input for our house intercom system?

So my house’s intercom system randomly started working this morning. Yes, we were all woken up to a sharp static-y radio noise. ghost house ftw After that, I figured out how to use the radio and intercom system and now the songs reverberate all over the house and it’s pretty neat!

The important part – The radio switch can be either turned on to FM/AM/Aux. Except, I don’t see an Aux input thing anywhere! The set up is that there is a “master system” downstairs and then a small intercom/speaker thing in every room. If I could figure out an Aux input, I could plug in my mp3 player and then we wouldn’t have to listen to the radio! So how do I go about getting this Aux input?

Additional Qs – Is it like they have allowed the functionality but you have to install your own Aux?? This brings up another Q I have always wondered, some cars (like my mother’s Honda Accord 2003) has an Aux option but no input there either? What’s the deal? And how can I fix this?

Other info about this system – I am not quite sure what system we have because they don’t have the model # on it! And it might be in some of the house papers from when we bought it, but that would take way too much time to find. I know the company is M&S and I believe the one we have is one of their discontinued products

Thank you!!