Faulty Equipment and poor installation

I had an entertainment unit installed with GPS capable XTC head unit, amps, speakers etc. The amps were just thrown in loose and I had to take the vehicle back as the head unit also slipped out of its mounts...<br> <br> This was not the main issue, the GPS doesn't want to work for longer than 10min at a time. I've been back 4 times already and no progress! They finally agreed to refund me as they have no idea how to fix it, but refused to give back 100% of the unit's cost and only offered 80% back. This led to the manager saying \We've tried to help