There are two JBL systems actually.
The 10-speaker JBL Synthesis system it has speakers in place of the convex cargo mirrors.
The navigation equipped 4Runners also had a JBL system, but these have 8 speakers instead of 10. It has the sub and center speaker on the dash, but has the convex cargo mirrors.
However, 2003 and some 2004 navigation equipped 4Runners had the JBL Synthesis system with 10 speakers. These 4Runners do not have the backup camera.
eskimochaso is correct though, if you don't have the sub and center speaker in the dash, you don't have the JBL system. Which means you can upgrade system much easier.