Picasso Xsara Negative battery wire chassis point

[rustbucket]

I have been trying to locate where on my Picasso Xsara 1.6 HDi 90 HP (battery under passenger seat) the negative battery wire is attached to the body / chassis. Heynes manual says near battery (not much use). The positive wire goes under carpet / floor and onto terminal block under the bonnet. The negative wire goes under carpet / floor but nowhere to be seen . Do I have to take up carpet to find it?. Apart from curiosity I would like to check for corrosion as I have had a problem when the engine will not turn over on the starter. I have checked between battery negative and engine block for volts drop with a load on and is ok but would still like to know where it is. I asked in the Citroen garage but got blank looks.
Any help appreciated

[RichardW]

service.citroen shows it as a short wire, perhaps curving back under the battery tray.

[mick2me]

In case it helps anyone, the location is quite close to the Battery.
Ratchet spanner?