Under the passenger seat? I think some of the Mondeos fitted with a CD-changer at the factory had them mounted there. Only 6-disc though so I don't know if a 10 will fit.

'93 16V Mondeo saloon
Full factory RS setup lowered on 17s
K+N 57i, splitfire leads, chipped, full aluminised twin-exit exhaust
Sounds by Alpine, Infinity, Vibe, JBL
First Mondeo with indiglo gauges