Radio/Nav system VERY LOW VOLUME!!

[porlagran]

Hello, my friend has a 2005 sequoia, with the Navigation system installed,it is the factory one, and lately she is having problems with the volume. It doesnt matter if she turns it all the way up, it will always sound really really low. We were thinking that maybe it can be the amplifier, but taking it to the dealership just to check it is very expensive. I dont know if it can be just a fuse or something easy to fix. I dont know much about this cars since i drie a volvo, but i hope i can help her to safe some money and time. Do you have any ideas what can the problem be and how to fix it?
Thanks!!