When I bought my van I soon realised that the stereo only played through the left hand speakers so I pulled the stereo out and found the dealer when putting a UK unit in had had cut the blocks off the stereo wiring and scotch locked into the wiring loom so put stereo back and got new stereo wiring and an Iso lead . Plugged it all together and it still only played through the left hand speakers, ,left it at that whilst planning next step but yesterday found the left front speaker now not working , oh the joys of a mono sound system. Looks like I`m going to have hours of fun checking the wiring for breaks in all the usual places . lucky me
If I had all the money I ever spent on my Freda ............ I`d spend it on my Freda.
A common problem is a break in the cables where they pass thru the bodywork to the door by the hinge. It would be relatively easy to run a length of suitable speaker cable from stereo to speaker & just bypass the existing wiring.
Try a search on the forum, I'm sure this has been discussed before
The standard speaker cable is rubbish so just replace it. It will be quicker than looking for a break and you will have the right basis for an upgrade.
mine didn't come with any rear speakers so put two in the wiring was the easy part just ran it around and fed it under the trim that took about half an hour to do both sides, now the problem i had was getting the plastic speaker covers off and cutting the holes a little bigger as i put a set of 6x9 speakers in and the hole size is roughly 5.5x8.5 pain in the ar*e to cut out,
good luck though and remember the trim comes off easy enough