2001 Passat wagon; 2016 Golf Sportwagen
I concur that it is probably the wiring or speaker voice coils. If you can find and disconnect a convenient test point, it is pretty easy to check continuity with an ohm meter. The impedance (a.c.) across the voice coil should be about 4 ohms, which means the resistance (d.c.) that your ohm meter reads should be slightly lower. Don't worry about the precise reading -- it is either going to be a few ohms (good) or something like 100K or even "infinity," meaning open (bad).Many things.
If this is the Monsoon system, if the tweeters on the left still work, then the head unit (radio), and the wiring from HU to amplifier are OK. That leaves:
-amplifier (not common)
-wiring (happens; typical break point is in the flexible section from door to hinge post)
-speakers (most likely, due to age.)