Concert 1 with no amp on rear speakers

All A4's have ampe rear speakers, the amp is bolted to the left rear magnet.
So unless someone has replaced the rears the amp should still be there.
If it isn't then your best bet is to buy a small amp on ebay (like an Alpine 3520 or 3522) nothing too big otherwise you'll have to run a new power cable and wire this up to the pre-installed pre-out lead in the boot. You can pick these amps up for less than £20.

There is no way to wire up a Concert without using a rear amp as it only has one 2 ch amp built in for the fronts.