Stereo & Electronics - Sail panel speakers not working properly after new head unit fitted

Hi there - and yes I have read all the stickies about the stereo unit!
I asked this one over on the FirebirdV6 board and got no intelligent answers - seems no-one actually reads the questions before firing from the hip (such as "directly wire the speakers" and so on; read the bit that says the speakers are working) - so I thought I'd ask you guys, who seem pretty knowledgeable (ok obsequiousness overload over).

(94 Firebird V6 3.4, Monsoon 10-speaker system). Yup I know, ancient car, joke engine, but that's another project.......

I've replaced the GM head unit (CD player, graphic eq) with a JVC 4X50W head unit and spliced in the aerial, supply and speaker leads correctly (I've double checked over and again). There are NO spare wires and there is NO amp control wire as on a Camaro. When I swapped the old GM unit back in everything works correctly.
The head unit and the door and trunk speakers are working fine.
However; the sail panel subwoofers are not outputting a music signal but what I do get from them is a continuous "wooooooo", somewhere about the "A" below middle "C" and at very low (but noticeable) volume, so obviously the slave amp is doing something. The tone doesn't change with engine or rolling speed so it's not a bad decoupling problem.
Is this something to do with the onboard slave amplifier not switching on properly? I'm given to understand it's the speaker signal that activates it, SO, is the JVC not somehow giving the right signal level to switch the slave amp on?
Anyone any any clues?
Many thanks.
Baffled Malc.

A 94 doesn't have a Monsoon system although the old 10-speaker system was very similar. I assume you have a coupe (otherwise there wouldn't be 10 speakers). You are correct that there is no amp sensor wire - only convertibles had that wire. The amp should turn on when it senses speaker level audio signal from the head unit. If the amp wasn't turning on you wouldn't get audio from any of the other mids/woofers so I doubt that's the problem. Do you get any music from the sail panels at any volume level? Do you get any bass from the door mids? Have you tried the JVC head unit in another car or with other speaker setups (just to eliminate it as a possible problem)?

Ahh right - not having (or being able to find) a wiring schematic showing the routing to all the speakers I wasn't sure, I thought the sail panel subs were the only ones driven by the slave, didn't realise the trunk speakers were as well.
No, the head unit is fine in another vehicle, and the front/rear fade function works correctly. Also, the trunk speakers work fine it's just the sails not giving any music signal; the door mids work as well.
And the slave amp is not above the wheel well, because it's a 94 it's down by the rear seats and a ***** to get to.
Ho-hum, looks like I have a fun weekend ahead wiretracing!