The sub output on the head unit is probably unbalanced meaning that it has a live side and a grounded side. The original output, and that which the sub amp expects to see, is balanced so neither are grounded they are a positive and negative output. You may find that disconnecting the ground at both ends, and only using the Orange wire will drop the level but will cure the hum.

Do you mean disconnect the black wire or the black and orange?

Orange/Black and leave the Orange connected to the centre pin of the phono output of the head unit.

Thank you. Sleep well.

Looking at RAVE it appears the DSP system uses Left & Right balanced feeds from the radio, plus the control signals. The DSP then drives the speakers in each door directly. The subwoofer amp has a balanced feed from the DSP. If you're using an after market radio, then the DSP becomes redundant, and the new radio needs to feed the speaker directly.

For the High-Line system with door amplifiers, then you have several options:

1. Remove the amps & feed the speakers direct from the radio.
2. Feed the door amps & sub using the "resistor attenuator" bodge. Some peeps find it works, but other don't. Probably depends on the radio model.
3. Feed the door amps & sub using line-outputs. This can also cause hum, whine or buzz, but also depends on the radio grounding.
4. Add balancing transformers between the radio line-outputs & the door amps. No hum, whine or buzz.

In case it helps, this is how the balancing works. Balanced vs Unbalanced

FYI I used these transformers

He's replacing the head unit and (dead) DSP amp so is using standard speaker level outputs from the head unit and linking the in and out at the DSP amp location via crossovers. So it is only the sub amp that expects to see a balanced input but is being fed with an unbalanced signal, hence the hum.

Will one of those transformers balance the RCA pre-outs from the new radio to feed into the twisted pair wires?

I missed the second link initially. I've found loads of DRV134PA 2.0 Channel Single-ended Balanced Boards... complete with clamps for power and inputs and XLR outputs.

1) None of them are boxed.
2) I assume I need to buy a female inline XLR and connect the Orange wire to hot pin, the Black and Orange to cold pin and a Ground to the third.

They would but the pre-outs on the head unit are there to feed into an external amp which you don't have. One of these https://www.ebay.co.uk/itm/232575093087 should isolate the unbalanced sub output to the balanced input the sub amp expects to see.

I didn't expect anyone else to be up and edited my post... Doh!

So that's a simple solder inline on a bodged 2-wire from the radio on its input, to the Orange and Black Orange on the output, then smother in PVC tape? :-)

Yes, the two wires, or centre wire and screen, from the phono output to one side of the transformer with the Orange and Orange/Black to the other side of it.

I'm nocturnal, so can often be found posting late at night but not early in the morning (unless I'm in a different time zone that is). I regard 4am as late last night not early this morning......