do as ive said and report back,theres no point guessing till you do
if you dont have a multimeter what you could do it is attach a fused(make sure this is done) wire direct from the battery, to the battery AND ignition feed on the radio,you should be able to hook it up on the jvc 301 as itll have the bullet connectors to do so. then turn it on,if this works then one of the feeds is flailing,its very common with older vauxhalls but havent seen it on a seat.
if you want to really test all your speakers then get a 9v square battery and pop 2 peices of wire into 1 set of speaker wires (with the headunit disconnected) and then touch the wiring to the battery terminals (no more than 2 seconds at a time as youll damage the speaker)-if you here a lovely crackling sound you know it works but like i say this is unnecessary,do the above.