Without looking at it I cant say for sure but sounds like the retro fitter has mismatched the red (LIVE) and yellow (ACC) wires, redoing this should solve the radio issue. As for the aux, is it connected at back of stereo, should be a blue connector.
Ebay sells twhat you need.
http://www.ebay.co.uk/itm/Audi-A2-A4-A6-ALLROAD-TT-S4-S8-Car-Interface-USB-Mp3-AUX-In-Adapter-High-Quality-/282311272519?hash=item41bb102847:g:8p8AAOSwEppUNfok
This will give you aux and usb connectivity. For a bit more you can get it from UK seller on Ebay.
I've just bought my Audi A3 convertible. I would like to use my USB pen to play music. The car has a concert stereo??? An aux port near handbrake and a 30pin iPod socket in the glove box. How could I use my USB to play music?