New Audi owner . Headunit problems

A few problems i wanted to put online to see if i can get any advice.

First off i have a few problems with my navigation system .

I cant get the sat nav to do anything , Its stuck on saying please insert navigation DVD disc but even after inserting nothing changes . Another problem is i cannot get my Aux to work it just says external audio source is active at all times . With or without connection . I can only assume somebody has maybe messed about with it in the past and miss placed some wires? Or i need a new system replaced . And for a 3rd problem i also have a light on the dash at all times telling me theres something wrong with the seatbelts .

I am all the new to this so any advise would be grateful

Most importantly i would like to know where is a good place to take the car to sort these problems . I have an Audi a3 S Line black edition 11 plate..

Thanks....