Couple of ways to attack this one lads.
Basically the HU is going to have to come out for a start.
Now you have your original loom im assuming, however over the years maybe things have been altered off of this.
In your original loom is it the plug or have things been bullet connectored off it?
If its bullet connector thats a bonus because fault finding becomes less hassle!
going on
Bullet connectored firstly
Check all connections are tight and crips are touching cable not sheath, then move to offending speakers and check there.
Once thats been done if all is sound swap around rear speaker output on the HU loom bullet connectors with the offending front ones and see if it works.
IF SO
there should be a channel down on the HU (Front), they do offer repair services, usually resistors go, but can be quite pricy to have done £80ish is a common charge I believe
IF NOT
check continuity of cabble with a continuity tester (This can be performed by connector blocking both + and - ends of the cable speaker end and one prong each on + and - loom end. tester will buzz if continuity of cable is present.)
If it doesnt buzz and show continuity you have damaged cable somewhere and is adviseable to change if poss

This also means you can repair the cable however it may pay to run newer insulated cable without joins
Hope this helps lads

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