If it is wired in already, chances are it'll be on the left or right side in the boot. Not sure about the saloon version, but the Weekend has a storage compartment on either side. Take the cover off and look there. You could also just take the plastic panels off, then you're bound to find the wire there (or not).
HTH,
<font face="Verdana">--
<font color="maroon">'98 Fiat Marea Weekend ELX (1.8 16v).</font id="maroon">
- Kenwood MP3 head unit
- De-Restricted standard air box
- NGK VX Platinum plugs (BKR7EVX)
- Toyo Proxes T1-S (205/45) tyres on Stilo 7-twin-spoke 16" alloys</font id="Verdana">