Greetings!

Unfortunately, it involves checking the connection. Since you are getting some signal at maximum volumes, there is some kind of connection. But I expect that either:

a) the aux-in wires are loose or barely making contact
b) your factory radio is not sending out line level signals
c) your factory radio does not have proper line outs, and this was somehow rigged by your installer.

You would need to physically take a look at the connections. Does your factory head unit have a CD or tape? Do they show the same volume problems?