My rear speakers don't work unless I put the volume at 30 and even then they just cut out every so often for random periods of time.

Anyone know how I can fix this?

Replace them.

Could it not be wiring?

To both, two wiring faults?

There's the possibility of the head unit being faulty too.

Extract the stereo far enough to get to the connectors at the back, and see whether you can make the fault(s) come and go by waggling the connectors while it's playing.  I repaired one for a workmate where the solder joints of the connector pins inside the unit were cracked by vibration/strain over the years, and needed re-flowing.  Took quite a lot of disassembly to get at them, mind.

I'd test the speakers first, it's by far the most likely fault.