Usually mounting holes are centre and with another 2 just behind rear seats headrests L & R in parcel shelf area, mounting points are covered by flat push in black plastic that hold down parcel shelf--------------- trouble is without having a good pic it looks like your rear speakers have been mounted where they used to be !!!
Have a look in your boot up under the rear parcel shelf and see if you can see any pressed/spot welded speed nuts, the same thing happened to a friend of mine (not lancer) as they were removed when speakers cut in, if so then an alternative mount-ing spot will need to be found in the rear floor area using a 6mm x 50mm x 50mm square steel plate under boot floor pan for bolt to go up thru, if a suitable reinforced area cant be found.