I was gonna do the double din conversion, but came across a A6, TT head unit, it's marked Concert II and is single din, has a CD player, but don't know if it holds more than one, has anyone put one in a B5 will it wor?k

 

If the wiring harness will plug in, it should work. Look up how the model CD player you found hooked up to the old car. Check that cars wiring harness with yours and make sure the wires match up. Should be fine depending on the year that the car came out of. Worst case senario you try to plug in the wiring harness and it doesnt fit, or it fits but doesnt work. Trial and Error.